Okay, I threw a Halloween party for a client last year and the following was a list of suggested "couples" costumes which I've updated with a few, more current additions. Hope you find something that works, the party was actually a huge success, people felt a lot more comfortable when they brought their mates with them and their costumes matched.
Princess Leia and Han Solo
Bumblebee and flower
Danny and Sandy from Grease
Doctor and Nurse (except switch the unfortunately obvious roles)
Romeo and Juliet
Angel and Devil
Harry Potter and Hermione Granger
Fred and Ginger (are you willing to rent tails, if so it's a pretty easy costume for the guy and the girl gets to look fabulous)
Bathing Beauty and Lifeguard
Cop and Criminal
Tacky Tourists (this one was a big hit)
Jolly Green Giant and Little Sprout
Ozzy and Sharon Osbourne or Jack and Kelly Osbourne
Bonnie and Clyde
Sonny and Cher
Scarlett O'Hara & Rhett Butler
Lucy & Ricky
Raggedy Ann and Andy
Cheerleader & Football Player
Tarzan and Jane
Fred and Wilma Flintstone
Bam Bam and Pebbles
Cowboy & Saloon girl
Aladdin and Jasmine
Belle and the Beast
Esmerelda and the Hunchback of Notre Dame
Mickey and Minnie Mouse
Peter Pan and Wendy or Tinkerbelle
Tarzan and Jane
Dorothy and the Tin Man, Cowardly Lion or scarecrow
Wonderwoman and Superman
Superman and Lois Lane
Batman and Catwoman
Ken and Barbie
Morticia and Gomez from the Addams Family
Dalmatian and Cruella DeVil
Biker couple
Little Red Riding Hood and the Big Bad Wolf
Cowboy and Cowgirl
Playboy bunny & Hugh Hefner
Elmo and Cookie Monster
Big Bird and Grover
Flapper Girl and Prohibition Gangster
Mr. and Mrs. Clause
Cheerleader and Football Player
Shrek and Fiona
Bride and Groom
Adam and Eve
M&Ms
Bunny & Carrot
Greek god and goddess (this one seems appropriate)
Men In Black
Gilligan and the Skipper
Professor and Ginger
James Bond and Bond girl (you can decide on which Bond, which decade)
Sid and Nancy
Kermit and Miss Piggy
Popeye and Olive Oil
Caesar and Cleopatra

good luck!