No, not stupid ... your receiver is configured to support a single rear centered channel if you do not indeed have 2 speakers.
I don't think combining the signals would yeild any benefits and may actually cause problems.
I run a similar setup with a Yamaha receiver (RX-A2010) without issue. You will need to go into the receiver menu and update your configuration, or let YPAO do it. That will likley net the best and most trouble-free result.