Why not double up on PEX cinch clamps?
I do not think it would be a good idea to double up on the clamps.  When you are clamping PEX onto a fitting, you want to try to get the clamp near the middle of the fitting, ideally between two ribs.  With two clamps, it wouldn't be possible to do this.  

To ease your trepidation about the staying power of the clamps, do a test clamp on a fitting.  Let it stay overnight, and then try to get it off.  You should find that it's very strong.  If you ask me, the PEX + clamps is tougher than copper and solder.