They've definitely talked about giving the contestants scores before. Maybe they need to have them enter the scores into a keypad, so that it's locked in before the discussion. Then use the scores to break the ties. That way Nina and Michael can't bully Heidi and guest judges into giving them their way, though I wouldn't put it past Nina and Michael to give really low scores to a particular designer, just to make sure he/she had to go.
And, of course, there would be issues about who went down the runway first, as an early design might be given a high score only to be followed later by an even better design, which, depending on how they handled the scoring, might not be able to be given a score significantly higher than the previous design.
Maybe they just need a human tie-breaker who isn't involved with production, so that the winner isn't chosen for dramatic effect as opposed to his/her design abilities.