I actually just faced this yesterday after getting a mini to go with my Roamio Plus.
Here is my hardware/network set up:
-cable modem -> Apple AirPort Extreme -> hard wired to location A -> switch -> Roamio
-cable modem -> Apple AirPort Extreme -> hard wired to location B -> switch -> Mini
At first, the mini would simply not "see" the Roamio. I called support and they said something about the devices needing to be in the same subnet. That was Greek to me so I did the following: removed the switch at location B and connected the mini directly to the wall jack (hard wired to the AirPort Extreme).
Once done, the mini saw the Roamio and I was able to complete the set up without issue. I then re-inserted the switch into the chain at location B and the mini continues to work - live tv, recorded shows, netflix, etc.
It remains to be seen what would happen if I lost power and one or more devices were reset. But for now things work fine. Strange that the additional switch initially blocked the connection between the devices though...