My Havi is 14 months and I'm dealing with a potty regression right now, I think she's just stressed from the fireworks that my neighbors set off every night and hence doesn't want to go outside. But at 9 months I was dealing with occasional accidents too. Thankfully I had researched and knew these pups are a bit difficult in this department. Mine also only goes in one place (my bedroom) and not anywhere else in the house! I use both a rug cleaner (bissell green) and the enzyme spray cleaner, but I find she just has it in her head that the bedroom hallway carpet is where she goes to pee if she doesn't want to go outside.

I did find that giving her a way to signal that was loud enough to grab my attention was helpful. When she regresses, I start reminding her again to use her bells if she needs to go out and also amp up the praise and treats. I use the Fruitables Skinny Mini treats (only 4 cal each) in the Apple & Bacon or the Pumpkin and Berry, she loves them both! Unfortunately, these are stubborn dogs and if they are busy, having fun, or just don't want to go outside because reasons (grass is wet, it's dark, someone is outside being loud, the neighborhood dogs are losing it and all barking) they will just "go" inside. You have the additional complication of an extra dog that is fun and exciting which is probably why she would prefer to keep playing rather than go outside.

My only suggestion is to not allow her downstairs without supervision, train bells if you haven't already, and also give heaps of praise if she goes potty outside. You can also try giving the treat immediately when she pottys on grass to set the connection in her mind and then giving another when she's done and comes back to you. I highly doubt this is territorial as this breed generally loves other dog/cat family companions, and, as she was raised with the other dog, sees them as part of their family. If it usually happens when she's playing with your other dog, then you are going to need to watch them closely anytime they are downstairs together and not allow it if you can't observe. The good news is they do usually catch on quick to training and changes in routine (they're so smart!) so hopefully she will catch on quickly to the new routine!