Does stat bonuses stack in BG2?(like you Draw upon holy hight twice,or use something like Girdle of Giant strength then draw upon holy mighy,etc)

Experiment :)

IIRC, DuHM stacks with pretty much everything, and since your casting level is not limited… Well, you can pretty much have an unbeatable character. Furthermore, I believe that Bhaalspawn's DuHM stacks with Cleric's or Paladin's. And, once again, since your casting level is not limited, it'd be possible to have some STR of 38 with Girdle of Fire Giant strength (STR 24?), plus two DuHMs at 21st level. Cool, ain't it?

Erm...all stats have an upper limit of 25.

iirc Armor of Faith stacks, though that is a bug. Similarly with Bless.
All stats have an upper level of 25.
You cant use DUHM twice and hope it stacks.
As far as plain old Clerics bashing some poor sods head in is concerned Holy Power followed by DUHM is the best.

GoFGS is str 22