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.