If using external drivers you can hook up multiple drives to a single step/dir output. I run a machine with 5 axis motors but use only 3 external drive outputs on the duet (2 motors on Y, 2 on Z, 1 on X) for a 1000mm^2 machine. Could hook up four drivers for the Z axis using this same technique.
Might have to shift the logic level of the output signal depending on the external driver. I just use this one: https://www.filastruder.com/collections/electronics/products/duet-expansion-board
As others have mentioned it's easy to hook up a SSR for mains power to the existing heater outputs with no change. Could even connect multiple SSRS together without an issue if you have multiple heated areas. Just have to be cognizant that you can't PWM most SSRS very fast when running AC through them. Probably want to run the bed heater in bang-bang mode (which it is by default) or cap the frequency to the ac mains (50/60hz).
