I thinking finding a design you both really like and getting it in the same place would be a good idea.
You should try looking at the types of designs that symbolise eternity, something that’s saying “i never end”. Even a word that best describes your relationship would look awesome..
The back of the neck is always a good place for male or female so, if you deside on an image make it relatively small.
If it’s wording (I’m no expert) maybe have it diagonal so it doesn’t make the neck look wider.
What ever you do, don’t get names!
Saying that, I would get maybe a first letter.