Found this double cobble gen while browsing the MC forums. This is not my design, but it is a pretty cool one. You will need about 50 cobble, but just allocate a stack for good measure
Step 1: Layer 1
Create a 5x5 cobble layer, then mine out the blocks to create the shape show below
Create a 5x5 cobble layer on top of the first, then mine out as shown below
Place a water source block in each corner as shown below. DO NOT PUT YOUR LAVA IN THE CENTER HOLE IN THIS LAYER.
Create a 5x5 cobble layer on top of the second layer then mine out as show below. There should be a 2-block-deep hole in the center.
Place your lava source block in the top space of the center hole so it flows down
Done! You can stand in the water or in the missing corner to collect the cobble.
I can make a video if needed, but I think this is pretty clear.
Not 100% efficient, but more efficient than other designs.
A little glitchy. Sometimes cobble will be thrown outside of the box, but just make a loop around and you'll pick it up.