I found lots of stuff of mechanically laminated beams which was interesting but not very useful at the moment.

I did find a formula for Fb for a two layer non laminated beam which basically seemed to deliver the same required Fb rating for two 8x8s stacked on top as each other as for one 8 (width) x 16 (depth). Which is odd but maybe not too surprising.

Would failure due to horizontal shear be less of a factor given that the two members in the stacked beam are free to slide past each other?