Use a keyboard which can switch it’s MIDI channel on the fly. The Keystep does this, as does the X-Keys.
Then you have each box on a different channel, and switch between them from the keyboard.
one more option, without additional hardware: there is the possibility to set a midi track on dt and dn, which then gets send out via midi out,and to the midi in of the next device. if this track is selected/active… maybe you can try this first: first dn or dt and a4 as last in the chain. to keep it simple maybe, try with keyboard and two devices at first
btw. there are many similar topics in here already, so please also try the forum search…