I could see this being a good idea if they'd voted for BOTH primaries... but to me the choice itself should give some indication of which party their voting for. ie. if they marked Obama or Hilary, but leave Romney, McCain and the other Clowns alone their vote should be counted, and visa versa.

My gut reaction is that election officials spend far more energy on figuring out ways to toss out votes, than they do on any other part of the processes.