Making Salmon en croute. Can I cook two trays, one on top of the other at the same time and have them cook appropriately?

I would add to what June said, to stagger them if possible on their separate shelves, so that the top one is not completely covering the bottom one (one a little further to the right, the other a little further to the left); when shifting them, again, keep them staggered so they brown more evenly.

It should work if you switch them about half way during the baking so each of them have a turn on the top shelf.

I've never tried this specifically myself, but I have cooked other pastry-based dishes on several levels successfully. It works well if you have a fan/convection oven. I wouldn't try it with a regular oven. And be sure to space the trays out evenly (ideally, same amount of space above, between and below). Hope that helps !