How to connect an extra pair of speakers to a stereo?

[RealityClash]

Hey all,
Today my neighbour came over saying he wanted to get rid of a pair of speakers, so we decided to see if we could hook it up to our stereo. Our stereo is a Sony STR-NX1. Currently we have the 2 original speakers which are plugged into the 'front speaker' outputs.

On the back of our stereo there are outputs for a Left Front, Right Front, Left Rear, Right Rear. All four of these outputs are for the black and red naked cables. There is also an RCA output for a sub.

I am wanting to plug the 2 speakers from my neighbour into the 'rear speaker' output plugs, but when I do this I still only get sound in the front/ original speakers. The only way I can get them all to work at the same time is by having 2 speakers plugged into each of the 'front speaker' outputs as they are just naked cables and do not have any sort of plug on the end of any of the speaker wires.

Are there any negative effects from doing it this way?
Does anyone have any ideas why the rear outputs would not be working?

Thanks

 

[americanaudiophile]

If you hooked up 2 pairs of speakers to the front speaker connections and you stereo did not blow up then you could leave them connected that way. You still might have a problem if the stereo overheats when playing at a loud volume. It may shut off until it cools down or it might be damaged.
To get the rear speaker outputs to work you would have to select a surround sound mode rather than stereo. You may also have to go into a speaker setup menu in the Sony and set the rear speakers to on.