Best and Worst idea ever. It all depends...
With the right coding partner this can be the best thing ever for productivity (I've had a couple of such partners). With the wrong partner, it is a disaster (I had one of those, too). Lots of little things, from programming/testing style to quirky personal habits can make or break the arrangement. So it should *never* be forced on programmers, but should definitely be encouraged when it occurs organically. As the previous commenter noted, it's actually a lot like a marriage (or dating)...