I can't help you on the 920, but for the 520, you just go to the power meter settings and adjust the crank lengths there.

With the older Edge's, you would have a profile for each bike (thus, each bike remembered its specific crank length). That was probably a bit easier then this is now, but on the 520, it is certainly easier to forget.

It's actually pretty annoying how they have it setup on the 520 (and I assume other newer models). As mentioned, you have to delve into the menus to access the PM settings and change crank length every time you swap the pedals. Very easy to forget. Would be nice if they allowed you to tie the crank length to an activity profile and then just let you choose a different profile for each bike/activity. Changing activities is both easier to remember and fewer button pushes.

Well that makes decision easier then- I'm sticking with same crank length for both bikes. As easy as it may be to swap these pedals bike to bike, I want to minimize time spent on all of this.

Yup! On the 510 it's easy - the bike profile's are different so you just set and forget. The 920 is a pain, you have to change it manually before every ride.

I realized I have a 510, not a 520. So, if I use different crank lengths and set bike profiles to reflect that fact, would those differences be properly picked up by P1 or would I need to manually re-adjust crank length each ride?

Just make sure you have the correct bike selected at the start of ride and it will set your crank length for you.

Thanks. That what I was hoping.