If the water demand of the 2 zones you want to wire together total less than the supply, AND both zones have the same type of heads (all rotors or all sprays), yes you can.

The Hunter Pro C is expandable. You would have to go and buy a module and that will expand it by 3 more stations. It should only cost you around $23 to $25. If you double up stations sometimes you can burn out that station in the controller.

Here is a link that might be useful: diy site

Be careful doubling up on the controller. If they share the same field common, it will cause feedback and burn up the transformer. Buy a module. Just find a Hunter distributorship in your area, go and ask for one. They should have plenty in stock. The big three Irrigation distributors, Horizon, Ewing and John Deere Landscapes all stock Hunter pro-c modules.