highlight the images you want and say print images
In the window that pops up there is a little flower thing in the lower left hand corner, when you click it you can say "New contact sheet preset".
You should now see a preview of the images taking up most of the paper. Simply change the border options to get the correct size. You might have to print a few times to get it perfect. Then simply name that preset something like "two 5x7" or something you can remember.