What happens when two people try to lift each other simultaneously?

I've done this with my sister, where we grab each other under the arms and heave, and we struggle and sweat, but nothing happens.

My thought is that I have to lift her weight + overcome the force of her pushing upwards off the ground, and I don't have it in me, but I'm struggling to understand exactly what's going on there.

Maybe I'll rig her up to a pulley system and see what happens.