A while ago a forum member posted link to a SRAM PDF that explained why their double chainrings shift so well. I just can't find it anymore and if anyone knows what I mean would appreciate link - was a really good paper.
Anyway, main point is that they keep the chainrings in a ratio of 3:2. Meaning the small chainring has 2/3 of the number of teeth of the larger one. In my case a 28/42 etc. They said that gives 4 natural overlap points where the chain glides on the ramps from tooth to tooth. It seems most if not all their double cranks follow that 3:2 rule. I know they had to say their design is good, but it made sense. I can attest that it shifts very smoothly (better than my old 3x crank with 22/32/44).

Obviously other ratios also work and as attested above it seems to work well. but if you can, sticking with the 3:2 rule may be good. Obviously this limits your range.