It will depend on the main receiver.
If it has a record out (which is not affected by the volume control) that would be perfect. If it has a zone 2 output that would work too. That would allow you to play different sources in each receiver if you wanted that.
If it has a preamp output that will work but the volume setting of the main receiver will affect the volume of the second receiver.
If the main receiver doesn't have any line level outputs then you could use a speaker to line level converter. That would connect to either the A or B speaker terminals of the main receiver. One pair of speakers would be in parallel with the converter which is fine.
If you don't already have two receivers you could get a 4 pair impedance matching...