You would need to have all the Tivos' remote address in System Information set to a different address for this to work, 0 is just an address that uses all 1-9 addresses. The Roamio needs the remote address set also.
It is best to use a remote with a 1-2 switch to set beforehand.
If instructions are needed, go to:
Be sure you are out of the System Information screen on both Tivos, or they might get changed.
Now, on to setting the Roamio remote.
Press and hold Tivo + C to switch to IR mode. Set the remote using Tivo + Pause (from above) to set the address of the other TiVo. Remote should blink red on button presses. Done. You can now control the other Tivo under this mode.
To get back to controlling the Roamio, Press and hold TiVo + D. Remote blinks yellow under RF mode remote use.
The end result is roamio is under RF control (TiVo + D) and the other is on IR (TiVo + C) and you can just switch between the two.