@BillW,
A multi party system can only develop when you adopt a two-round voting system. The first round of voting allows a variety if profiles and opinions to be tested, while the second tound pits the two candidates with the most votes.
In effect now, the primaries act as your first round, and the general as the second round of election, but this means the parties control the first round, and can to some degree protect their vested interests there, i.e. by excluding or weighting against certain candidates.