You can’t really. There is only one volume setting for the amp, and you only can adjust the balance between left and right channels. Although you can connect 4 speakers, it’s essentially designed for two.
Perhaps a work around is set the amp to play mono, so there left and right channels play them same audio, then connect the new speakers as left channel, and the older speakers as right channel...or vice versa. Then you could use the left/right balance to balance your two zones. Down side is you give up stereo.
edit: or you could put a volume control switch between amp and your two speaker pairs.
Four speakers will be OK if they are 8 Ohm. Not if they are 4 or 6 Ohm. You may need to consider an impedance managed speaker switch with volume controls.