You can do it without having to have a switch (in a 2x12), you just need one jack socket that has switched contacts. You then wire it so that one speaker is daisy chained off of the switched contacts, and when you plug into one of the jacks (the non-switched one), you have a mono cab, but when you plug into both, you have stereo :)
(i'll try and draw a picture up whilst i'm in a lecture this morning)