bugger.. when i built my front portch i used two 2x8's with a 1/2" PT spacer (cut so the water runs off it) and sandwitched the three items together. but what i found was that the tops of the boards were not perfectly flush.. so i had to shim in some 1/16" galvenized metal (some Simpson strong tie thing) so joist would sit on both beams.
so my backyard shed i decided to build beams differently.. just sandwitch them directly, no spacer, and use water proof membrane to protect the seam. but i find the same problem! i used my body weight to make sure the bottom was flush because i want the weight transfered to the posts evenly.. but now in SOME areas the beams are not flush on the top. what to do? shim again? hit it with a sander? don't worry about it?
"if you can't be handsome, at least you can be handy" -Redgreen