First off, this is the old, tube 1200S, right? If it's the newer, Fender-era 1200S with the solid state output, then none of this is applicable.

The output transformers on the old Sunns have three outputs: 4, 8 and 16 ohms. They had only two output jacks, though. the way they were wired, there was one main output, either 8 or 16 ohms, which was determined by the cabinet the amp was sold with. The other jack was wired to the next lower impedance, either 4 or 8 ohms. It was also wired such that, when a plug is inserted, it switches the other jack to the lower impedance. So, if you have one 16 ohm cabinet, you plug it into the 16 ohm jack. If you have an 8 ohm cabinet, you plug it into the 8 ohm jack. If you have two 16 ohm cabinets (total 8 ohm load, in parallel), no problem. You plug one into the 16 ohm jack, and the other into the 8 ohm jack. The internal wiring puts both cabinets in parallel, connected to the 8 ohm tap on the output transformer, and everything is fine.