I'm trying to do the part where you ride the two motorcycles to break the wall and I am having no luck. Everything I've read seems to say to switch back and forth between characters and make them duck. Everytime I reach a certain point, though, the person on the bottom motorcycle teleports up to the bike I'm on. What am I doing wrong?

Right, jump on top of the bottom cycle and press down and A to summon your partner and make them stay.

Then jump over tot he top motorcycle, which makes you start moving. Press the X button to switch to your bottom character and quickly crouch. Press X to switch back to your top character and crouch quickly to avoid
another lowered ledge. Then, press X again to switch to your lower characterand press down again to crouch under another lowered ledge. Finally, press
X again so you are controlling your top character. An elevated ldge should be closing in on your location. When you see it approaching, perform a very low jump by tapping B to barely clear the ledge and land back on the motorcycle
again.