I have 2 Streams on the same network and you just have to go to settings/setup in the IOS app and wait 30 seconds-ish to have it go through the steps to get it setup to stream in the app... You'd have to switch to the stream that is paired with the tivo at it's location to Stream from that particular tivo.
This obviously is easy to do when in the house. I haven't tried swapping between Streams outside of the network, I'm not sure it would work... Ok, just tested this part, using my phone as a hotspot and you can indeed switch between streams from outside of your home network. One part done.
But, theres a couple question marks left:
Whether the app can deal with tivos on the same account being on separate networks?
Whether you can change between streams on two separate networks?
-Roamio Pro/Slide Pro
-Stream (Gotta go unplug the sucker one of these days!)