4 or 8 Ohm speakers

I would suspect it has the power, at 120wpc. you say it has zone controls too.

If the zone controls afford powered signals to a second area, the consideration then would be speaker imp and length & conductor size of the speaker cables themselves.

Keeping up conductor size should help and there are some other threads currently asking the same thing which have links that might help you figure out what is best for your circumstances.

A high speaker impedance is likely going to be best for this effort..

It's not having enough power, it's about too low of impedance. Driving your main room and the second room all at 8ohms may be too low for the receiver. Check your manual.