Just do it level by level. Just take the better of the bonuses for each.
For example: (I don't have my books to check yours, so I am making one up)
1. Wizard1 / Rogue1
Wizard = +0 BAB, Fort +0, Ref +0, Will +2
Rogue = +0 BAB, Fort +0, Ref +2, Will +0
So for first level, you have a +0 BAB, Fort...