hm interesting.

Are you using the same cranks and a "mirror image" bottom bracket? I think chain line would be affected if you're not, and one side may bind based on how the wheel is lined up. In other words the chainring/cog combination that is better lined up works. Because the other one doesn't line up, the other direction binds.

Lining up both chainrings with both freewheels might be difficult. I'm sure it's possible, I just don't know how some of the newer BBs would work. Track racers occasionally tried using two chainrings and a double sided hub in order to have a more efficient drivetrain. The one I'm thinking of also had a "dump" feature so that once up to speed, the racer could dump one of the chains to make the bike "lighter" and more efficient. More a gadget than anything else but hey, at least they tried it.

good luck with the double freewheel fixie,
cdr