In the account of the transfiguration in Luke, we see this description of the event:
“And behold, two men were conversing with him, Moses and Elijah, who appeared in glory and spoke of his exodus that he was going to accomplish in Jerusalem. Peter and his companions had been overcome by sleep, but becoming fully awake, they saw his glory and the two men standing with him. As they were about to part from him, Peter said to Jesus, ‘Master, it is good that we are here; let us make three tents, one for you, one for Moses, and one for Elijah.’”
So, not only do the apostles see the transfiguration, they hear the conversation, sufficiently enough to understand the context of the discussion. Scripture doesn’t record what was said, so we can’t know if Jesus addressed Moses and Elijah by name. But, given the context, is it too difficult to suggest that they would know the two most important human figures in Judaism, based on their discussion with Jesus?
Moreover, “Moses and Elijah” are, quite literally, “the Law and the Prophets”. The allusion wouldn’t have been lost on the apostles – this, in a very real way, is Jesus interacting with the Jews’ whole belief system in the Old Testament!
So no – without any ‘cop-outs’, I think it’s reasonable to suggest that the apostles were fully aware of the identity of the two men conversing with Jesus!