If you use a common ground between boards, you can couple the signals using transistors or logic ICs with open collector/drain outputs. With 4000 series chips, you could even use high value resistors ( 100K ) to connect the output of one IC to the input of another. For best noise immunity, you would put the resistor on the board that has the input IC and you'd place the resistor right at the input pin. That way you don't have to worry too much about one board not being powered and the signal lines trying to power the dead board via the CMOS input protection diodes. You still have to keep in mind the "default behavior" of the circuits when only one is powered down and the signal lines are dragged to ground.
If you can't use a common ground and decide to go the optoisolator route, here is an example of one with a open collector logic level output. You'll still need a current limiting resistor for the LED input:
https://www.fairchildsemi.com/ds/HC/HCPL-2601.pdf