According to this review of the new 60197 passenger train you can only have one Bluetooth thing talk to the new LEGO battery box at a time.
The new Power Up system as it currently appears in this set is very rigid. One remote or device can be used to operate one battery box/receiver, which in turn can operate one train motor. All of the flexibility of the old system seems to be gone.
I'm hoping to put a train in my office before Christmas and it would be great if multiple people could install the app and change the speed. Is that too much to hope for with LEGO's current evolution of this technology? This seems like a normal thing to expect for something that uses Bluetooth, but the review above makes me doubt that applies to LEGO. Has anyone tried this?