tape out the back. print all the chests, flip the screen and register the back. tape out the chests and go. the shirt loads the same, just make sure you put the front on the front back on the back, etc.
I put the design for a left-breast print at the same basic height that I would be putting a regular design's film on the screen, but have it a couple inches to the left side (if looking on the back of the screen, where you would be taping the films, shirt side of the screen)
You would put it in the bottom left rotated 180 degrees I believe. Just put the shirt on your pallet and lay the heart down. put a screen on top and see where it needs to be positioned.