If the driver for your MIDI controller allows multiple running instances, it’s not a problem to map Renoise songs to it.
So you could have the master level for each song mapped to two sliders, A and B
Notice also how Hitori does it in the video: certain songs have certain reserved spots on the controller
(mapped controls are saved as part of the song).
If you controller does not support this, you could feed your controller to some virtual midi port (of which there are several…
I personally tend to use loopMIDI by Tobias Erichsen) and split the controller input into multiple outputs (one for each Renoise).
If you need to fiddle with multiple Renoise configurations, this is possible too.