If you divide them up into 2 sets of 300 levers, you'd have 300 bits for counting the votes for each candidate (which a max number that probably exceeds the amount of stars in the milky way).
All the voters have to do, is pull or push on the the correct set of levers to increment the votes of their candidate by exactly one. If it's done wrong, the final count might be off by a few quadrillion, but that's within margin of error.