Hi
I am no expert but I would confine her more if she is going on your couch or bed etc. I had a yorkie many years ago that used my house as a toilet for a very long time.It is a hard habit to break once they start.
Maybe you could block her in the kitchen? There would be only her pee pad on the floor instead of the rug and couch .Hopefully she is just doing this because of the changes going on with another dog and another person coming into the picture. Maybe she is marking?
A lot of good advice probably better than mine is on the way
Good luck with her.
Just wondering how you know which one is doing the mess if you are at work and leave them in the living room?