The ones with the rectangular tray have everything in one piece so you can't double up on the coffee. The one I had this weekend with the round pods doesn't really have space for two pods. I've tried it, and that's one case where I had an overflow problem. The coffee gets compressed more than the water flow can handle. The real mess comes when I've put coffee into the place where the pod goes, tearing the pod open, discarding the supplied coffee, and just using it as a filter.

I would occasionally use two packets of the kind that they give you to put in the flat-bottom (standard) filter basket, but double strength bad coffee is still bad coffee. And I've tried using just half as much water with the small pods, but oddly, it's still too weak.