I have two profiles and I want to merge messages from both (only the
ones that aren't duplicate) into one. Any suggestions?

Most of the messages are duplicates, but there's about a month's worth
of mail that's in one but not the other. Basically, I had a hard
drive fail (profile A), restored and used a profile from backup
(profile B), then managed to resurrect the entire drive's contents and
went back to using the profile (profile A) on the restored drive. I
still have profile B with a month of mail that I now want to merge
back into profile A.

Worst case, I could just forward those messages to myself. Is there a
better solution? I can't drag and drop them from one Thunderbird
instance to another, can I?