On the personal front, clear two-way communications about assumptions, expectations, family plans, personal financial goals and responsibilities, career aspirations, further education, areas of compromise and many other non-glowing-sunset-beach-walk-romantic topics, are critical to the survival of the relationship. These are the areas where couples often founder. Compromise and acceptance are critical, so that the unresolved issue doesn’t seep back to poison the relationship.
On the professional front, with the same Service, the usual policy is to “do their best to support co-location of dual military couples,” which could mean duty stations in the same state or on the same coast, but make no promises. Sometimes the choice is “sure, we can give you orders to where he/she is, but that will be a lateral step in your career, as we don’t have a X career milestone billet available there for you, but we do have one in (opposite coast).” No promises for unmarried couples.
As for inter-service dual military, you will have to compare your career paths and milestones, look for areas with joint bases, determine when you can handle tours apart, or is one of you going to be primary and the other a “follower.”
It can be done. Most military women marry military people. DH and I were both Navy, and spent several tours apart, but it just made us stronger together. All of our disagreements related to the items I named above, which we had to work though. Any couple can tell you assumptions and expectations can widely differ between two people
Final bit of advice. If someone bites off a “Fine, it’s fine,” to end a conversation, it is probably not.