Is it ok to have both amps connected with separate cables but only having one amp powered up at a time? My concern is added impedance from the other powered down amp. I’m guessing this isn’t a very good idea no matter how convenient it would be.
Your guess is correct, but for a more serious reason. The outputs of two power amps should absolutely never be connected together, whether the connection is directly between them or is via the speaker terminals, or serious damage can result. That applies even if both amps are never powered up at the same time. For example, if a connection exists between the amps when you are using the S200 it may at times apply 40 volts or more to the unpowered output transistors of the smaller amp, in a manner that is likely to "reverse bias" their base-to-emitter junctions much of the time, and in doing so greatly exceed the maximum reverse bias the transistors are rated to be able to withstand.
A Google search will turn up various devices that would enable you to select which amp is connected to the speakers, while leaving the other amp unconnected. If you want to consider one of those, be sure it is rated to handle the power capability of the S200.
Also, if the smaller amp happens to be a tube amp, keep in mind that a tube amp that has output transformers (which means most tube amps) should not be powered up without a speaker or equivalent resistive load connected to its outputs, especially if it is being provided with an input signal. And in most and perhaps nearly all cases a switching device would leave the unselected amp unloaded.