That's a tough spot to be in- my wife and I are in the middle of that right now too- sort of. Last year, we tried to get into the same schools. She applied for religious studies, and I applied to Philosophy programs. She got into Rice- one of her top choices- and I was waitlisted there. I ultimately did not get in. I don't think it helped much for me to tell the philosophy dept that my wife already got accepted to Rice. In retrospect, it may have been counterproductive. They were probably more hesitant to let me in because they may have felt like they only did it because my wife was there, and not because I was a good candidate on my own merits. Now we are considering the possibility of going to school in seperate places for a while.
Be ready for that possibility. If you both want to get an advanced degree and things don't work out in terms of going to the same school, you can increase your options by going to seperate schools. It's not ideal by any means, but more people do it than you would think. Overall though, I would advise against mentioning the fact that your significant other has also applied. You want to stand on your own as a good applicant, rather than potentially introducing an element of emotional manipulation to the admissions commitee.
It's not going to be easy. If you're really lucky, maybe you can go to the same school. If not, be ready to live in seperate places, or have one person wait.