What are your plans or goals for this?Mud truck, street/strip, DD?
Not trying to talk you down from this build, but why twins, Cool factor? Also, the why the Allison? It is not rated to hold much more HP/TQ from stock. If you have not purchased one already, get a late model one, at least one 2005. They can handle almost 600ft lbs. The 6 speeds a bit more.
As for gearing, the Allisons are lower than the 4L80e. The 1st gear is low, so getting you moving with the snails will be nothing. The bad, its geared low enough, to make it almost pointless with this set up. 2nd and 3rd gears are close, but still lower. I'm not sure of the max RPM rating, but I'm sure you'll have to shift at 5K. Remember they are made "mostly" for the diesel folk and not designed for turning high RPMs. You should probably get away with 3.73 gearing and have good low end and top end.
