[Snake]

What is the bug? It reduces momentum cost by 1 when you do a basic hit while under the airborne status effect, and that's it.

Sky Mantis probably works like "if [Airborne] and obj/Sky Mantis, 3M = 2M", not like "if [Airborne], (3M - 1M per obj/Sky Mantis) = 2M", I assume. So wielding one Sky Mantis on each hand won't change the effect to make airborne strikes cost 1M.

Still, you're welcome to test and see if it really reduces momentum the other way (subtractive), since this is just an assumption.