If I’m RPing with your multimuse blog, I am 100% down with RPing with both character A and character B in the same thread.
If, for instance (and picking generic names), the thread started with Charlie, and Victor is logically nearby, Victor is welcome to butt in. He can pop in for conversation, walk in on Charlie and I, or if your muse for Charles is being stubborn and it’s situationally appropriate, Charlie can excuse himself to the bathroom or go get takeout or answer a phone call in the other room while the story picks up with Victor instead.