GarageBand for iOS 13.3: how to make mono synths polyphonic?

I want to use one of the Bass settings from Alchemy Synth as a pad in its upper registers, but that's a problem, because the Bass sounds all act like mono synths and if you write two+ notes in a track to play, it picks one note and sticks with it. Other than the obvious workaround (add as many tracks as notes in the chord), how do you make a mono synth poly?