One digital output (from outside) connected to two digital output

What caution would I need to take to feed two digital input (to two different Due boards) with the same digital output coming from an anothet system 0-3.3)?

Connect the Grounds, don’t place them miles apart and don’t turn the input one off (then you will supply it via the inputs instead of the Vcc/Vin).

That list bit means do not power down either input system while the output system is powered.