What are you worried about - noise on signal lines (causing the wrong signal to be read/sent) or a voltage spike (all those heavy solenoids) ruining you electronics or a fatal short (like getting 24V on a 5V line)?
The ground has to be common - else there is no meaning to voltage levels. This is true also with several power supplies.
Unless you choose to galvanically seperate the two, in which case all signals have to go through optocouplers, and of course you have seperate power supplies.
The spikes should already been absorbed by diodes (the driver board contains electronics I see). Spurious signal on the input side can be compensated for in the software, it is akin to debounce a switch. To guard against a fatal short you need galvanic isolation, but if you have nothing more than 24V then you "only" need to replace the AVR microchip (or some driver chip you may have) in "the unlikely event" of disaster