Those RS masts are pretty hard material, so I don’t think pipe clips will squeeze them enough to clamp. So the two options I would try are to shim the joint, means pull it apart and put a thin strip of metal in to make the joint tight, or drill a small hole and screw a sheet metal screw to tie the two poles together. Remember that any movement in the fence post will be greatly exaggerated if you are looking at the top of the mast. Yes, I do think the support (fence) is too short for a 15ft pole, can you use just the 10ft one, or is that just too low?