I think you are describing an AB-Y dual looper, like this one:
http://www.brightonion.co.uk/aby-dual-looper/So you need three footswitches - loop A on/off, loop B on/off, both loops together on/off.
Here's a layout for one with a tuner/mute out as well:
You can omit the tuner/mute out section if you don't need it.