There is no evidence to suggest that Barbarika saw Visvarupa. But yes, Sanjaya did see the Visvarupa of Sri Krishna which is evident from his following comments:
11.9 Sanjaya said O King, having spoken thus, thereafter, Hari [Hari: destroyer of ignorance along with its conseences.] (Krsna) the great Master of Yoga, showed to the son of Prtha the supreme divine form:
11.10 Having many faces and eyes, possessing many wonderful sights, adorned with numerous celestial ornaments, holding many uplifted heavenly weapons;
11.11 Wearing heavenly garlands and apparel, anointed with heavenly scents, abounding in all kinds of wonder, resplendent, infinite, and with faces everywhere.
11.12 Should the effulgence of a thousand suns blaze forth simultaneously in the sky, that might be similar to the radiance of that exalted One.
11.13 At that time, Pandava saw there, in the body of the God of gods, the whole diversely differentiated Universe united in the one (Cosmic form).
18.77 O king, repeatedly recollecting that greatly extraordinary form of Hari, I am struck with wonder. And I rejoice again and again.
These can not be communicated or recollected unless are seen personally.
I think he was shown this because of the grace of the Almighty Sri Krishna. If Sanjay was not allowed to see this, the whole world would have been deprived from some beautiful parts of Gita and may be many of us would not be able to believe in it if only what Arjuna saw was recorded.
So it seems another play of the ever- playful Sri Hari.