I have two bicycles (OK, two bicycles with Look Keo pedals). One is touring bike with an ultegra triple crank (older, probably 2006 vintage) and the other one is a Roubaix SL2 with Ultegra 6750 crank.
I set the cleats on my shoes mostly using the touring bike since I also commute to work on it. I probably put 2/3rs of my total mileage on the touring bike. When I use the same shoes on the Roubaix with the double/compact crank, the cleats are slightly off. They are workable but they feel slightly off.

First question. Given that all other measurements (handlebar drop, size, seatpost, saddle height) are identical and both bikes have a Fizik Aliante installed, I am guessing that the difference I feel is due to the q-factor of the triple versus the double crank. Am I on the right track here?

Second question, can I shim the compact crank pedals by using stainless steel washers by about 2mm on each side so that the I minimize the q-factor difference. My pedals instruction manual says that up to 2mm shims on each side is OK.

Thanks for all your help.

The only thing that could make them different is the crank arm length and how far the pedals are away from the cranks. Never the less your feet should contact the pedals in the same spot on both bikes.

They are both 172.5 mm cranks. I think there is a 4-7 mm difference in the q-factor between the two cranksets though.

when i use the same shoes and cleats on several of my bikes that are fitted with compatible pedals and otherwise set up the same, which my bikes are, i always assumed that anything that felt different, whether right or wrong, was attributable to the differences between the bike's frames or wheels or tires or the materials they are made of. i don't think i am alone here.

the only bike that i regularly ride that could exhibit a noticeable q factor discrepancy would be my MTB with a triple. and if it did i would ignore it rather than attempting to widen all my other bikes in order to create uniformity. but if you actually have a manual that addresses legitimate pedal shimming tolerances and you only have to fuss with one, i say go for it.

OTOH, i think i might first try deciding which one i liked riding most. if only to avoid the possibility of owning two bikes with a uniform but sub-optimal q factor.