double stack on a bun

When two individuals need to defecate so bad, they use the toilet at the same time. One sits on and a little forward of the first person's lap, and they both relieve themselves.

Crikey, Bruce, I've got to shit so bad we're gonna have to double stack on a bun!

So there I was, minding my own business, when Jane busts into my stall screaming, sits down in my lap, and starts shitting right through my legs. I never thought I'd have a double stack on a bun!