My cab is currently wired at 4ohm mono/8 ohm stereo. I'd ideally like to have the mono ohms at 16 so I can run two cabs, but keep my stereo capability.

As it stands I have two jacks with no switch. The white (left jack), runs the cab in mono. The black (right jack), runs one speaker. If I connect two heads through each jack, they get one speaker each.
Any ideas?

EDIT: Short of buying one of these, which would undoubtedly make life easier