I hadn't noticed until you pointed it out. A "Carryall" is broadly defined a "vehicle in which two seats face each other" and I find a golf cart like vehicle called that for sale. More searching revealed a bit of an autobiography in which the writer says he and his brother bought a carry-all...
I can't speak for query letters but since there are any number of SF novels which have co-authors, I don't see that there would be a problem. If you are concerned, though, why not make up a single name using both your real names and submit the query that way to the literary agent?
Have you tried Rochelle Alers, Joanne Skerrett or Brenda Jackson?
ReShonda Tate Billingsley might be a possibility, too. I'm more of a mystery and fantasy person, so I was going to suggest Stephen White or Olivia Butler until I read the rest of your question.
Good luck finding something you like!
Patricia Windsor has some really good ones. I thought The Christmas Killer was excellent but all of hers are good. Well written and exciting, touch of the supernatural.
Nancy Werlin's The Killer's Cousin was also good. A young man is staying with his cousins after getting out of jail for...
I'd recommend Making of a Woman Vet by Sally Haddock. Came out in 1985 but still very good. She talks about her time in vet school, learning the ropes, treating patients, etc.
Another good one is If Wishes Were Horses: The Education of a Veterinarian by Gage.
Herriot's books ARE...