The 24 Supply will tolerate 12-24V. All that supply is used for is the bias to the optocouplers on the isolated inputs, and the feed to the high-side of the (low-side) switched (MOSFET) outputs. The actual voltage is largely irrelevant (you'll get less current through the LEDs on the isolated inputs.... but they'll switch reliably). My personal view, and I'm ready to be corrected, with the isolated inputs that there's enough immunity introduced to not warrant a separate 24V supply and given space constraints and pre-existing 12V supply - I'd use the 12V supply. You know how the wobbly-wobblies work, Kitwn, let you come to your own conclusion.