I'd say make the rules as simple as possible.
1 character, hard mode. Everything else is permissible.
The second character is dead, only except when it is necessary to use to complete plot/quest items. I'll probably give it a shot
-e
Well, the problem is that you can make a Lone Wolf, Glass Cannon, Sneak 5, Leech, and the game won't be particularly difficult.
There are lots of mechanics which can be abused, so you have to specifically rule them out.
