[matese]

Lets start with Abbie and the car. Do you use a booster seat in the car for your babies. If not I would try that to eliminate the nervousness and panting. If they can see out of the window a booster seat may help.
I can understand just taking taking Minnie Mouse, less stress on you and more enjoyable with a quieter pup.
However, you have to work on Abbie's issue of barking when out, take her out more often, alone, without Minnie Mouse, she may need more socializing around strangers. Let her great and sniff people, take her to Pet Smart, Pet Supplies, PetCo. Put her in the carts baby seat where she is up, level, can see people. The more interaction she has with people may calm her down, not get over excited. (I never let my pup walk in any stores, he is always in the carts baby seat. I put a blanket in it to prevent him slipping through the metal bars). This would have to be a consistent thing, try it for a month twice a week or more if possible, the more times she can be around ppl the better. Then bring Minnie Mouse on these trips.
So to answer your original question, I see no harm in taking only Minnie Mouse out for the day where you both can have a peaceful, quiet day.
I don't want to lay a guilt trip on you, but, it is not something I would do.
Work on Abbie's nerviness, socialize her more, take treats with you, it can be her kibble, when she starts barking, tell her no barking, as soon as she stops quickly give her a treat. Treat must be given the instant she stops barkng, even if she stops for only a minute. She will learn no barking is a good thing because she gets a treat.
If you do not have a booster seat I suggest getting one large enough for both pups.
Keep us posted.