You can probably recover the 1st session with some tool (I don't remember programs to do that now, but with a bit of search you'll find them).
I don't think it's a good idea to merge Nero6 and Nero5.5 sessions.
Nero6 is probably buggy. I wrote this a month ago:
[i]"I was able (unfortunately, because I had taken the CD to a friend without checking) to burn a disc with a lot of file-entries in the index while the actual file was missing.
I mean the list of files in the explorer was correct but several files weren't actually burned.
How? I had a (5.5version) NRI multisession compilation with a lot of MP3 files, and I copied all the entries to a new (6 version) NON-multisession compilation. The files in grey colour in the old compilation (which shouldn't be written again in the multisession copilation) weren't actually burned in the new compilation. The colour in the new compilation was black, so nothing made me suspect that they weren't going to be burned.
Maybe it's only a problem with old NRI compilations, but I'm not sure. I think I've seen a couple more of strange things, but I'm not sure if it was my mistake.
Anyway, Nero6 burned an incorrect CD, so it's a bug. "[/i]
I didn't see any message about not using Nero 5.5 compilations with Nero6 multisession, and I doubt that Ahead warned about this... :Z