I travel a lot and a friend of mine watches my cat, she has a cat too and they love each other, when I bring her back home it is just the two of us and she meows a lot and does not always use her litter box. Should I consider getting another cat?
I am worried she will not want to come back home with me.
You should consider that, and you should just have your friend go to your place to see to what the cats need, without transporting the one back and forth all the time.
Likewise, your friends cat is probably doing the same thing, missing her friend, and should consider getting another cat as a companion for her cat.
Another solution, is that one of you take the friends cat, and the one that gets the cat could buy a replacement cat for the donor, maybe even two cats. Sounds o.k. to me. Good luck in your decisions.D