Do you need two pocketwizards to trigger one flash?

My set up is a D300 and One sb-800 with a white 36" umbrella. Should I get pocketwizards? I am using a Cactus trigger now, but i want something more substantial. But do I need to buy 2 PW's to set off one flash? and then another PW for each flash? It will cost me almost $500 in triggers alone if i want to use two flashes?!