Originally Posted by Jeff Wills

Yes, the double chainring crank would use a shorter spindle than a triple crank. BITD, we’d try different spindles until we found one that worked with the frame and crank we had. If the chainring contacted the frame we’d try longer. If there was too much clearance, we’d try shorter.

A rough guess would be a spindle 4mm shorter than the existing one. That’s assuming the difference is only on the right side- which is a tall assumption.

If it were my bike, I’d install a cartridge BB that’s s little bit shorter than the existing BB. As long as the chainring and crank arms clear the frame you’ll be OK.