The question of whether or not the Báb and Baháʼuʼlláh physically met at Khánluq, near the village of Kulayn, has long been a subject of controversy—but elsewhere Baháʼuʼlláh Himself, and beyond Him ʻAbduʼl-Bahá, attests to the strictly spiritual nature of this encounter.
What follows is my translation of an excerpt from Nosratullah Mohammad Hosseini, Ḥaḍrat-i-Báb (2nd ed.), pp. 325–326, in which Mohammad Hosseini quotes these attestations to demonstrate that this meeting was decisively spiritual, not physical.
See also this valuable resource on the subject; I am grateful to Farhan Yazdani for bringing it to my attention.
All footnotes are mine.
[With reference to the following account from Nabíl-i-Zarandí:]
I have heard Mullá ‘Abdu’l-Karím recount the following incident: “My companions and I were fast asleep in the vicinity of the tent of the Báb when the trampling of horsemen suddenly awakened us. We were soon informed that the tent of the Báb was vacant and that those who had gone out in search of Him had failed to find Him. We heard Muḥammad Big remonstrate with the guards. ‘Why feel disturbed?’ he pleaded. ‘Are not His magnanimity and nobleness of soul sufficiently established in your eyes to convince you that He will never, for the sake of His own safety, consent to involve others in embarrassment? He, no doubt, must have retired, in the silence of this moonlit night, to a place where He can seek undisturbed communion with God. He will unquestionably return to His tent. He will never desert us.’ In his eagerness to reassure his colleagues, Muḥammad Big set out on foot along the road leading to Ṭihrán. I, too, with my companions, followed him. Shortly after, the rest of the guards were seen, each on horseback, marching behind us. We had covered about a maydán when, by the dim light of the early dawn, we discerned in the distance the lonely figure of the Báb. He was coming towards us from the direction of Ṭihrán. ‘Did you believe Me to have escaped?’ were His words to Muḥammad Big as He approached him. ‘Far be it from me,’ was the instant reply as he flung himself at the feet of the Báb, ‘to entertain such thoughts.’ Muḥammad Big was too much awed by the serene majesty which that radiant face revealed that morning to venture any further remark. A look of confidence had settled upon His countenance, His words were invested with such transcendent power, that a feeling of profound reverence wrapped our very souls. No one dared to question Him as to the cause of so remarkable a change in His speech and demeanour. Nor did He Himself choose to allay our curiosity and wonder.” 
The available evidence suggests that Fáḍil-i-Qáʼiní  believed the Báb’s matchless joy on that occasion, as well as the special grandeur and majesty He exhibited, to have stemmed from a physical meeting He had had with Baháʼuʼlláh, which he surmised to have taken place at Khánluq (near Kulayn). 
In a Tablet bearing the signature of [Mírzá Áqá Ján] Khádim—dated 23 April 1887 and addressed to the Hand of the Cause of God, the martyred Mírzá Muḥammad-ʻAlí Varqá—Baháʼuʼlláh states:
… He Who heralded the light of Divine Guidance—that is, the Primal Point [the Báb]—attained to outward seeming the honor of meeting [Baháʼuʼlláh], albeit concealed from all. 
On its face, one might infer from this utterance of Baháʼuʼlláh, as well as the contents of the Táríkh-i-Badíʻ-i-Bayání,  that the Báb did meet Him. If, however, we turn our attention to other statements from Baháʼuʼlláh—as well as the utterances of ʻAbduʼl-Bahá, and also the contents of Nabíl’s Narrative—it will become clear that the Báb and Baháʼuʼlláh certainly did not meet, and that the encounter mentioned in the Tablet to Varqá was a spiritual one that occurred in the spiritual worlds, similar to Jesus Christ’s encounter with Moses, and Elijah’s experience on Mount Tabor.
In a Tablet revealed for Shaykh Káẓim Samandar, Baháʼuʼlláh attests to the fact that, after the Báb received a letter delivered to Him by Mullá Mihdí Kaní and Mullá Mihdí Khuy, He expressed His great longing to meet Baháʼuʼlláh. However, Baháʼuʼlláh makes no mention in that Tablet of an encounter between the two of them ever having taken place. 
While describing the events which took place in Ámul, Nabíl-i-Zarandí quotes Baháʼuʼlláh’s reply to the chief mullá of that city:
… although We had never met Him face to face, yet We cherished, none the less, a great affection for Him. 
The incident at Ámul was contemporaneous with the Báb’s imprisonment in the mountains of Azerbaijan, so one can infer from Baháʼuʼlláh’s statement that a meeting between the two had not taken place up until that time.
With regard to the question of whether the Báb and Baháʼuʼlláh ever met, Ávárih, the author of Kavákibu’d-Durríyyih,  has written:
There are those who believe that Baháʼuʼlláh was among those Who met with the Báb en route, and the source for this account has been attributed to Ḥájí Mírzá Jání.  However, based on what we can glean from reliable histories and credible accounts, the Báb and Baháʼuʼlláh categorically did not meet, and this account which has been attributed to Ḥájí Mírzá Jání is entirely baseless. 
Upon reading this section of Ávárih’s work, ʻAbduʼl-Bahá wrote the following statement in the margin:
There was definitely no meeting [between the Báb and Baháʼuʼlláh]. 
Moreover, another statement in a Tablet that ʻAbduʼl-Bahá wrote for a certain Jináb-i-Shukúhí of Shiraz dispels all doubt on the matter, and makes clear that the Báb and Baháʼuʼlláh never met physically. ʻAbduʼl-Bahá states:
The Ancient Beauty [Baháʼuʼlláh]—may my life be offered up for His loved ones—did not to outward seeming meet His Holiness, the Exalted One [the Báb]—may my life be a sacrifice unto Him. 
Lastly, we can also infer from one of Baháʼuʼlláh’s statements in His Súriy-i-Haykal that the Báb very much wanted to meet Baháʼuʼlláh, but that His desire was never realized in a physical sense:
Had the Primal Point been someone else beside Me as ye claim, and had attained My presence, verily He would have never allowed Himself to be separated from Me, but rather We would have had mutual delights with each other in My Days. He, in truth, wept sore in His remoteness from Me. 
 Nabíl-i-Zarandí, The Dawn-Breakers (trans. Shoghi Effendi), pp. 228–9.
 A title of Nabíl-i-Akbar (1829–1892), a mujtahid who converted to the Baháʼí Faith and was posthumously designated by Shoghi Effendi as one of the nineteen Apostles of Baháʼuʼlláh. Nabíl-i-Akbar prepared a revised version of a history of the Bábí religion known as the Táríkh-i-Jadíd, written by Mírzá Ḥusayn-i-Hamadání. This revised version, to which Nosratullah Mohammad Hosseini alludes later in this excerpt, has come to be known as the Táríkh-i-Badíʻ-i-Bayání.
 In a footnote, Nosratullah Mohammad Hosseini writes: “[This is] according to the Táríkh-i-Badíʻ-i-Bayání (the corrected history of Mírzá Ḥusayn-i-Hamadání). Several decades ago, the present author consulted a copy of this work held at the Fatheazam Library (the library of the National Baháʼí Center of Iran)” (Ḥaḍrat-i-Báb, p. 336, note 17; translation mine).
 See here for a more comprehensive treatment of this Tablet.
 Refer to footnote 2.
 Baháʼuʼlláh, Ishráqát va Chand Lawḥ-i-Digar, p. 221.
 Nabíl-i-Zarandí, The Dawn-Breakers, p. 461.
 A comprehensive, well-written history of the Bábí and Baháʼí religions in Persian by ʻAbdu’l-Ḥusayn Áyatí. Following Shoghi Effendi’s accession to the Guardianship, Áyatí defected from the Baháʼí Faith and wrote scathing polemics against it, and he has come to be known among Baháʼís as “Ávárih” (“the wanderer”) as a result. Shortly after Áyatí’s death, Shoghi Effendi wrote the following in a cablegram dated 16 December 1953:
[Ávárih] will be condemned by posterity as being the most shameless, vicious, relentless apostate in the annals of the [Baháʼí] Faith, who, through ceaseless vitriolic attacks in recorded voluminous writings and close alliance with its traditional enemies, assiduously schemed to blacken its name and subvert the foundations of its institutions.
 The author of the Nuqṭatu’l-Káf, a very early chronicle of the Bábí religion notorious for its many deficiencies and inaccuracies.
 ʻAbdu’l-Ḥusayn Áyatí (Ávárih), Kavákibu’d-Durríyyih, vol. 1, p. 96; translation mine.
 This is an eyewitness account from the late Abu’l-Qasim Faizi, related orally to Nosratullah Mohammad Hosseini, who in turn conveyed it to me.
 Authorized translation taken from this page.
 Baháʼuʼlláh, Summons of the Lord of Hosts, pp. 51–52.