I am doing a duo for small clubs, wineries, restaurant decks in the good weather, etc. Acoustic guitar and percussion. (congas, hand percussion.). Can I get away using only one PA speaker?
It depends. If you are outside then having two speakers will help in two ways:
First, you need a lot more power outside than inside. The sound just goes and goes and never bounces off a back wall, so to get enough apparent loudness you usually need more watts. You also aren't getting any support from the ceiling, and grass usually absorbs more sound than a hard floor, so it's pretty much whatever makes it so the audience directly from the speakers. Even indoors it helps to have extra power available for headroom and just in case.
Second, more speakers gives you more coverage. If people are spread out left to right then having one speaker pointing at the left half of the audience and another pointing at the right have will help all them hear the best quality sound. One way to estimate the coverage of a speaker cabinet is to stand in front of it and look at the tweeter horn so you can see right down the throat to the tweeter driver. Walk left and right until the curve of the horn blocks the tweeter itself. When you can't see the whole tweeter driver, you're outside the effective coverage of the speaker. In practice it may be a bit narrower than that, but that's a rough estimate.
Note that you should pretty much always run a mono mix. You could have a stereo reverb return that is panned across the speakers, but otherwise all instruments and mikes should be panned center and/or your speakers should be fed one signal that is daisy chained.
I do plenty of 'little' gigs on one speaker. And it's often pointing at me more than at the audience.
Also consider this point: Some gigs have the audience sat down in rows with everyone wanting to hear every note clearly. On others you're a side-show. There need to be areas where those interested can get good sound. But there also need to be others where they can get away from it and talk! Full coverage isn't always the aim.
You can. But with two, the coverage is far better - more than twice! And if a speaker goes down - dodgy socket, etc., what back-up is there?
You never know how the speakers are going to be orientated, they may be 10m apart, or next to each other, but at 90 degrees. Using just the one seriously limits what you can do. On occasions, one may well suffice, so don't use the other - and that could in some circumstances, be your monitor.
So, for so many reasons, have two available, and sometimes you can even have fun with the stereo side of things, which will never be an option with one...