How wide is the 'main' listening area that you are going to be in the most? I would place them at either side of that area and no more than ~12 feet apart for good imaging, assuming you are not going to be beyond the 15 ft patio edge, the golden triangle rule still applies for proper imaging etc. But as this is for outside I suspect critical listening and proper soundstage are not required.
If you were to position them at the extreme ends you wouldn't have much of a sound stage with stereo music and it would not sound right, but you could run the system as a mono speakers and then have a speaker at either end, maybe 10' or so in from each end, this would fill the entire patio with sound quite nicely.
