it seems to be lifnoise's soultion would control the volume on your keyboards.
if you want to control the channel strip volumes inside of MS, then you just need to assing the virtual knob to the volume and then ADD another map to the secoind volume.
this is also a really cool feature if you Invert one giving you a cross fade or mix. to do it another step, set both strips Output to a patch bus and use that to control the over all volume while using the first control to control the mix between them.
OR you could put both to a Patch Bus and then just ontrol the Volume on the Patch Buss adding effects to both.
this is how you create a sub-mix of several instruments across several channels with different layers and effects and then 'master' patch bus effects. you can also set each to different pan positions and have some with MIDI efects like ARP, or maybe two different ARPs. its quite limitless.