If you're choosing between 100 and 120 I'd definitely say go for 120 too. When we got our first two piggies they were briefly in a 100 cage given to us by a neighbour but the difference when we moved to a 120 was amazing. Who would have thought that 20cm would make such a difference!
A couple of other things toconsider - a younger piggie although smaller actually probably needs even more room than an older one so they can run around and play (although I wouldn't dream of getting a smaller cage for older piggies either).
Don't think a double storey cage will give more room - I thought that would be the case but I hadn't realised not all piggies like ramps and, of course, you can't stop them both being downstairs, taking up the room, at the same time.
I'm about to build a c&c cage for the first time having had a double ferplast 120 that I "modified" by putting te two levels side by side rather than on top of each other (which has worked really well by the way).
I'd agree that a 4x2 c&c cage is a great size for 2 piggies if you can fit it in (each grid is approx 36cm x 36cm). It is also a good bit cheaper than a ferplast cage.
