Having that said, there are a few things that I would suggest to avoid and things to shoot for.
-What you like to wear and what you feel good in, that’s the most important.
-Coordinating outfits are great.
-Accents and colors that compliment each other are the goal. If she is wearing a particular colored top, maybe his collared undershirt can accent that color. If he is wearing a certain color, maybe she can accent it with her accessories.
-Colors that you know work well on you.
-The feeling you want to convey; lighter shirts on top give the pictures a more casual and bright look. Darker shirts on top give the portraits a more serious and thoughtful feel.
Do Not Wear:
-All black or all white tops, it does not create much visual interest and can make for a tricky exposure
-Busy patterns that may overwhelm the photo, and distract attention from the two of you and your expressions
-Things that are too reflective, some eye make-ups that have glitter in them, and things that shimmer will also reflect the camera flash. They can be distracting and can look a little weird.
-Try not to wear the same exact outfit as the other
Hope that helps!