How do I cook these two items in the oven at the same time?
Set shelves high and low.
Preheat to 400.
Put in the crab cakes for 5 minutes on top
Turn down to 387.5 ;-)
Put salmon in on lower shelf.
Check in 15 minutes.


Most ovens are not all that precise, really, and most recipes allow for that (your 5 minutes of variance on each item.) The top of the oven tends to be hotter than the bottom, so this works that by putting the cooler item lower.

For more drastically different items, you cook one, wrap it up, set it aside to hold, cook the other. You might undercook the first a touch before holding it and put in back in the oven briefly before serving, you might not, depending what it is and how much "hot out of the oven" is an important part of serving it well.