For your collision box, add an additional 1 px collision box at the base of it. If this one overlaps the normal one (someone is standing on top of someone else), it pushes one or both rapidly off to a side until it is no longer overlapping.
If you don't want to use two separate boxes, you can also use is overlapping at offset condition to check below.
https://game.capcom.com/cfn/sfv/column/131422?lang=en
Thanks, that article gave me something to think about. I think you've led me to a functional solution for now using overlapping with offset. But I would like you to clarify what you mean by "add an additional 1 px collision box at the base of it". Would that be a single pixel or something at, say a pixel wider than the collision box? Would that be a solid box?