My understanding is that generally when the sprinkler system is set up, there is only one common wire shared among all sprinkler solenoids. Also, assuming the expansion board is placed near the main controller, even if you have two separate common wires, they would be fairly close so both can be inserted into the main controller’s COM terminal (there are two ports on the COM terminal).
If you place the expansion board far away from the main controller (and thus use your own extension cable), then I can understand the reason to carry the COM wire from the main controller to the expander. But we made a conscious decision to leave COM wire out of the extension cable, because COM wire carries 24VAC, thus a mis-wiring or plugging in extension cable incorrectly can cause instant damage to the circuit board. It’s safer to leave it out of the extension cable.