As I understand it, each member of Congress (Senators and Representatives), has a given number of slots at the academy(I've heard 5 for Representatives, not sure for Senators), and for each open slot there, they can nominate 10 candidates.
So, having more than one congressional nomination would increase your chances for an appointment by making you eligible for more than one of the pools of slots that each member of Congress has for candidates, in addition to making you eligible for an appointment off the National Waiting List.
Does that help any?