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If I want to have two jacks available to drive two cabinets at times and only one cabinet at other times, how do I do this ? If I wire them from the output transformer my only choices are series and parallel. One way would get me 16 ohms and the other gets me 4. I want it to always be 8. Id the only answer that when I have 2 cabs plugged in I need to set the selector switch to 4 or 16 ? My son's Marshall has two jacks and it will accept one or two 8 ohm cabs, and if it's just one it doesn't matter where it's plugged in. How does this work ?

The jacks would be wired in parallel and you would either have to have an impedance selector switch or configure your cabs accordingly. Obviously the selector switch is more convenient.

If two 8 ohm cabs are plugged in, then it's a 4ohm load and the switch would be set to 4. One cab, 8 ohms. Two cabs in series in one jack, 16 ohms.

thanks Kevin:

after doing a little more research the flexibility which my son's amp has is probably due to the fact that it's a Mosfet powered amp and it seems that impedence matching isn't the issue that it is with tubes and output transformers. I have 3 holes in the back of the chassis, so I think I'm going to go with having a speaker out that's fixed at 8 ohms, a line out jack so I can send the signal to any other "boost" source and that will leave the third hole available for a footswitch if I want to go with channel switching in the future.