From looking at the CGX info on Voodoo Labs site, it sounds to me like you can activate any number/combination of loops at any time. They even have a diagram of how to hook up stereo loops at the same time as mono loops (and 12 other setups).
I haven't looked at any of the other options (Carl Martin, etc.) and I'm not implying any endorsement of a specific product, but it does look like the CGX will do what you want and I imagine that there are other options, too.
