Are you using photo software? I use Phoptoshop, but a good freebie is paint.net`. For printing I use 300 dpi, You can create a blank A4 sheet at 300dpi (maybe save it as "blank") then open the two pictures you want,size them at , say, 7" x 5" at 300dpi. Then you select on one of them "all" then copy, then paste on the blank. Do the same for the second - it will be on a second layer, then move them to one is at the top and the other at the bottom. Then print.
HTC U12 Plus review: Hands-on