In every other music app I use on my Mac - DAW or standalone instrument - my two MIDI keyboards (one a weighted piano, the other a synth-style one with aftertouch and controller knobs) work simultaneously. That's how it's always worked in Studio One v. 1-3. I recently upgraded to version 4, and now as I add a new track, each one will respond only to one controller or the other. Even if I change which one it's supposedly assigned to with the yellow drop-down at the top left of the instrument window, it continues to play with only the controller it "chose" when I dragged the instrument into the track. Both controllers show MIDI input in the indicator at the bottom.
I've been using Studio One for years, and I'm tearing my hair out over this.