Aim for the jugular

Line the apples up next to each other, I guess. Sort of like taking a single slice through multiple carrots on the cutting board at once. Harder to do with apples given their shape, but I'd the knife is big enough, or you're counting a slice as "single continuous motion" then it is probably valid.

I can't think of any other physical solution that isn't a joke, so this is the most probable puzzle solution. In a D&D setting I might require a slight of hand check with a very low DC to pull off the single slicing motion.

You line up the apple and cut both at the same time

Cut 2/3 of both apples leaving 2x2/3 segments and 2x 1/segments (2/3 cut in half for those of you who struggle with fractions)