Paralleling the two speakers will, I believe, half the impedence, so the amp will be seeing 3 ohms nominal and dips could bring it much lower. Look at the impedence curves, but this might put a real strain on the amp.
It will be a 3ohm load which is fine if played at low volume. But keep in mind it takes more volume for outdoor speakers due to the lack of boundaries. A speaker selector would let you turn off the speakers not being used and protect the amp.