The old "vote for the worst opponent" line has been going around for as long as we've had democracy. There isn't anything to stop you doing that, if that's what you really want to do with your vote, but you'd be sacrificing your ability to vote for your favoured candidate in your own party. Either way, you are going to have to leave the election of one of them to the winds of chance, so it might as well be the other guy and may the best man win.

I presume that you aren't allowed to register for both parties, or at least that a kneecapping awaits you if you do. Even in America.