I have read that you can run RCA cables from one receiver to the other. Main unit Out to second unit IN. Is this possible to do safely? If yes will I be able to run 8 speakers at the same time? Both receivers have A/B settings. I don't have an issue if I end up with two volume controls. Thanks
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 speaker selector instead. That will protect the receiver from damage.