The mortal wound is additional to other damage, so you would resolve the normal wound first, if applicable, then the mortal wound, which would spill over in the case of single wound models. Not super clear, but should still work out the sensible way.
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From a midnight sky, there is a searing flash, a boom, a brief moment of destruction, and then it is gone.
Kabal of Lightning Strikes - Project Log
Drukhari damage output analysis