As they're for the patio, I suggest stereo isn't necessary, so use the centre channel output. You should be able to select a mono mix of L&R to the centre channel. Put the 'speakers in series to keep the impedance up. Any loss of damping factor will be insignificant for casual use.
If you can't select a mono mix to the centre channel, or you must have stereo on the patio, can you select L &R to the surround outputs?
S