You can use Generic Remote Device and set, the specific MIDI Message will select a dedicated track (bass). When the track is select, Record Enable is switched On (by default settings), and you can play the Instrument. Use another MIDI Message to select different track, to be able to continue with next sound.
Alternatively maybe also a “Keyboard Split” would be interesting for you. So you have the one VST on the upper key all the way on right of your keyboard and the other VST on all the other keys on the left.
Check this tutorial on how to do it.
I am not sure what exactly you are doing. There is also other ways you could realize something like this.