Two speaker pairs to a single amplifier.

Speaker selector switch.

I would like both loudspeaker pairs to play at the same time. 

Wire them in series.

- + - + = 8 ohm load (12)
- +
------
- + = 2 ohm load

in series = 8 ohm (12) YES
p
a
r
a
l
l
e
l
--------
2 ohm NO

8 to 12 ohms in series YES
1.5 to 3 ohms in parallel NO

Keep the runs (cables) as short as you can. Will it work? Yes. How well will it work? Don’t know.
May start a hole ne trend! :-)

Regards

https://soundcertified.com/wp-content/uploads/2017/11/speaker-wiring-2way-diagram.png

maybe this will help

https://www.zeroimpedance.com/index.html

this search

https://www.google.com/search?q=speaker+switch+with+volume+control&rlz=1C1SQJL_enUS881US881&oq=speaker+switch+&aqs=chrome.2.69i57j0i457j0l6.12266j0j7&sourceid=chrome&ie=UTF-8

revealed these among others

https://www.monoprice.com/product?p_id=38159&gclid=CjwKCAiAtej9BRAvEiwA0UAWXhTBZJoxHTRW3cjJIkVpg...

https://www.amazon.com/Improved-Selector-Distribution-Amplifier-Switches/dp/B000NJHAFQ/ref=sr_1_2_ss...=

Just in case it's not clear. If wiring in parallel (which you don't want to do because it will present a troublesome 2-3ohm load to your amplifier), you would simply connect both sets of speaker wires to the amp's binding posts.  To wire in series (presenting a more benign 8-12ohm load), connect the positive wire of speaker A to the amp's positive binding post. Connect the negative wire of speaker B to the amp's negative binding post. And the connect the two "left over" wires together. I use wago lever clips for this purpose. Standard electrical wire nuts is another option, as is "mash together and wrap in electrician's tape".