@beth Try behavior training before you add another dog. This is going to take a bit of repitition over the better half of a morning, and a few days of follow up training -- but it works.
Do whatever preperation you normally do to leave. Tell your dog that you will be "right back" (insert whatever command word/phrase you want to use). Then exit and lock your door. Walk around the corner of the building, then return. Praise your dog as you greet them. Wash, rinse, repeat, gradually increasing the amount of time you are gone. Start at a few moments, then trips to the store, etc.
This reassures them that you will come back,. but you are actually teaching your pup the routine ques. It alerts the dog that you are preparing to leave (without them). Slightly different for when they get to go with you. Your dog will learn the difference ("Hey, if she picks up the leash, I get to go, too!").
I (personally) use bribes as a tell-tale. I give my pup a kong filled treat, or a frozen neck bone (her favorite) to chew on if she's staying at home. If I call her and pick up the leash, she's coming with. But she's ok, even on the rare occasion that I'm gone for 6 hours!
