I don't have a solution for you per se, but I found that using two fingers across two keys was suffice. (Button placement being important.)
I used two side aligned buttons on my gamepad to alternatively press with two fingers on one hand.
If that doesn't work for you, you could also try using two hands. eg. A finger from each hand to press two keys on the keyboard. (or thumbs if it's a gamepad). For me, the two hands method is faster and less tiring, but also easier to get out of sync. :satisfied
[Btw, I've only played through Easy and Normal so I don't know how far this goes.]