I'd be interested in this PBEM.
Can you have 2 leaders of the same nation in one game without any editing ? If you say so, kewl ! Even though you don't want to hear our choices, I'd like to represent France, given that Louis and Napoléon (in that order) were the French leaders I wanted for Civ4. I'd prefer PBEM if possible. Both leaders permanently allied for each player and all couples at war with one another sounds like the way to go indeed.
Never played a PBEM with 2 civs, but I've read it's a lot of fun splitting your research and production in two.