How do you calculate the BAB on the fused being created by the fusion power? If you fuse together two half bab characters does it make a full bab character?
You get the best bonus.
class features are pooled
this effectively means the fused being uses the better...attack bonus...of either member
The pattern is that bonuses of the same type do not stack, they overlap, and you get the best bonus available to you. If you have a Psion 20 and a Wilder 20, you get 10 BAB. If you have a Psion 20 and another Psion 18, you get 10 BAB. If you have a Psion 20 and a Fighter 14, you get 14 BAB.