I tend not to ever think it is a physical health issue. That is just me, and many, many expensive trips to the vet.

What is their absolute yummiest food/snack/treat? Try feeding them together- maybe two feet apart- that treat, and only then. That gets them to associate close proximity to one another with good things.

Next, try playing with them together. There is this toy called "da bird". It is ridiculous. It is like a short fishing pole with a feathery toy attached to it. I can't open up my hall closet without my cats running to the door, as they think it is 'da bird' play time. Play with them together. Again, they are expending energy AT something, and not AT each other...do this consistently.