You need an amplifier switch. I bought the Speciality AV SP 71 from Amazon US for
$85 aud.
I use my home theatre front speakers for 2 channel supplied from a Naim 82 pre and 180 amp. 7.1 channel supplied from Anthem 710 AVR. The front speakers are Linn Ninkas. Worked perfectly with no degrading of the sound.
https://www.amazon.com/Amplifier-Receiver-Speakers-Selector-Switcher/dp/B0796KGVXT