The only way I do a tattoo more then one time is
for friends, partners, siblings, brudis, colleagues in life or
anyone else who is up to make that bond between visible.
Ideas for partner tattoos:
dot-formations that built one image with your bodies (can be two or more people)
a creature that is concepted for each of you but melts in one when you are together...
I draw those according to your personal wishes
your partner's profile as hidden face in drapery/ optical illusion
or the classic one: the same motive two times