From what I've heard it works like this. You and your fiancee apply to residencies and they both rank you separtely. Then the computer finds the a city that you both ranked highly that the 2 of you matched.
Ex. Your husband ranks his 3 surg cities as SF #1, NYC #2, and Miami #3. You rank the city you want to do your anestesia in the same order SF#1, NYC#2, and Miami #3
Lets say that SF ranks your husband high but you low, NYC ranks you high but your husband low, and miami ranks you both high. If you were applying independantly, your husband would match @ SF and you would match @ NYC. However, since you 2 are matching as a couple any city that accepts one of you has to accept both. Since the only city that ranked you both high enough to match at was Miami, you'd be going to miami w/your husband.
Someone please correct me if I'm wrong, hope this helps