You only need one.
NO, you don't use it with spikes.
What it does is grabs the roap, locks when you push down with your leg, allowing you to advance upwards with a friction hitch (pushing it up). Sorry early, tired, not enough coffee. I use mine on my right foot, with the split tail system. Got a pic..
The rope sometimes will not slide through , but with technique it gets easier. I have also found that the higher up you get, the easier it slides through. I also tie my pantleg up for less interference.