That episode is messed up. The main problem I see is this: Supposedly, the Six-image and the Baltar-image (we can't really call them hallucinations) are appearing to Baltar and Six for the same reasons. I.e., it's not like Baltar has a Cylon chip in his brain, since Six obviously doesn't have a "human" chip in hers - no, both images appear for the same reason, whatever that may be.
Now, based on that premise, the Six-image is conclusively not a Cylon transmission, since the Baltar-image exists. Nor are the images mere hallucinations formed completely inside the mind of each person seeing the hallucination, since the Six-image has proven to have knowledge that Baltar could not have known.
So, they're not mental images, and they can't be pinned on the Cylons. Which leaves us with what? A powerful third-party influencing both human and Cylon for whatever reason? The God(s), perhaps? I think that's beyond what RDM would allow. Some other alien race? Also, not a direction BSG would ever go. Which forces us to revisit our original premise.
Ok, so let's look at what Baltar and Six have in common. They were both in close proximity to each other what hit by the blast wave. They both started seeing images of the other basically from the moment they were no longer in their presence (which coincides with the nuclear blast, unfortunately). However, the scope of knowledge differs from Six to Baltar - but then again, Six is a Cylon, while Baltar is human.
The only conclusion I can see is that at the moment Six died in such close proximity to Baltar, her transmission was partially intercepted by Baltar's mind. Somehow, her resurrection transmission took with it some of Baltar's personality, which manifests itself as typical and standard schitzophrenia. However, while passing through Baltar's personality, some of herself was left behind as well... a part that somehow has knowledge beyond Baltar's knowledge, perhaps subconciously connected in real-time to the resurrected Six on Caprica.