I bought both sets off Ebay. I bought a pair with the Sony speakers in them didn't sound good so I got the kickers to replace them. Right now I am running 2 sets. One set has only kickers the other set has kickers facing the rear and sony's facing forward. I installed a switch in the dual cans so I can switch between the front and rear facing speakers.
I had only the one set at first and was pleased. You could get away with one set. I got the second set so I could get the dual (Front/Rear) setup so I could hear the music while the boat is on the shore.
Make sure you power these with a good amp. Your head unit amp will not cut it. I'm using a rockford fosgate amp rated at 200 watts into 2 ohms. With the 4 speakers in parallel I have 2 ohms.