It's tricky because I truly don't think any pet store sells a cage large enough for 2 piggies. However, there is a really cool solution that just takes a little work on your part.
Very good directions are here:
http://www.guineapigcages.com/howto.htm
...but I'll share with you my experience as well.
First you need to track something down called corrogated plastic. I was surprised at how easy this was. It is just like cardboard from a box, but it is plastic rather than paper. It can be found at any sign-making company. I simply called one and asked if they were willing to sell me a piece. They did so happily. I want to say it was about $25.
You'll also need to find wire storage cubes. They are those things that provide storage in like dorm rooms, etc. It's a bunch of wire squares and connectors that you would fasten together to make stacking cubes. Well, in our case, we'll connect them to make a wall of your piggie cage.
You then simply score the piece of corrogated plastic so that it folds up to make a tray, only about 6 inches deep. I just fastened the sides with boxing tape - it has held for 2 years now. Then, connect the wire squares to make a wall around the inside of the tray. I had wire squares and connectors left over so that I was able to make a stand and a hayloft as well.
Piggies aren't very adventurous - they don't climb and they will not escape. There is no need whatsoever to have a lid, I promise.
Check out the website above for exact dimensions, tricks and good examples. Your piggies will end up with lots of leg room AND a lot less stink.
ETA: I see the picture here has the wire on the outside. I screwed up my measurements and could only fit the wire on the inside, but it works just as well and piggie can't chew on the plastic.