I haven't recently looked into what is currently on the market for this type of solution, but I know a few people who use "multi-zone" amplifiers (I believe they were by Niles and Russound) which have a stereo Bus input; this input then feeds all amp channels and would allow a second zone from a pre-amplifier to feed multiple speakers.
You could use a speaker selector switch, but I've also seen some of our older receivers come in with shorted outputs where the customer told us they were using a speaker selector switch with impedance matching and they believe it failed. If you are considering using a speaker selector switch with impedance matching, I'd look into one that has independent volume controls for each zone (maybe something like the Niles SSVC-4); First though, I'd recommend reaching out to a reseller of these type of products to make sure you find a device that will work safely, and properly, for you.
In regards to the sub:
While you could theoretically use the sub output for this, it isn't meant for this purpose and would be extremely kludgy. The sub outputs are linked to the main zone; this means different input selections, different volume controls, and having to manually turn the secondary sub off when using the main system so that it is not playing in another room when the second zone isn't in use.
If your sub has high level inputs and high level pass-thrus (like our Ultra-X12 did), you could run the secondary zone to your amplifier; have the speaker outs from your amplifier feed into the sub high level ins, and then have the high level outputs from the sub feed a pair of speakers. Then, set the low-pass filter on your subwoofer. It would definitely make a nicer use-case than using a main sub-output from the pre-amp for a secondary zone sub (even though you wouldn't have a true crossover in place).