Is there an issue with dual mono cancelling out other inputs?

[LooseSends]

I have input 1 and input 3 in dual channel mode.

On input 1 it's split between kick drum and side-stick as I have a home-made sidestick trigger.

On input 3 it's split between high-hat and cowbell (home-made).

All kitpieces work as you would expect, EXCEPT if I hit the kick and the cowbell at the same time in which case the cowbell does not trigger.

I've checked in the control software to make sure that this is actually the case since each one lights up (or doesn't in the case of the cowbell) when triggered. It only fails to trigger if both are hit at the exact same time.

I've switched cords around and switched assignments but oddly it only happens with the second assignment on input one and the second assignment on input 3.

For now I've split one of my cymbals with the cowbell, leaving everything else as is and it triggers just fine. Also, I double-checked the crosstalk, and no issues there.

Is there any reason for a trigger on one to suppress a simultaneous trigger on another input?