My son and I are building this together, but I wasn't sure if we both needed to register for the game jam. Do we need 2 separate accounts? Do we get 2 votes or are we as a team counted a 1 unit?

You can do it either way. If you want him to have an itch account, then he could join. He could then rate other games during the rating period. Or you can just stick to the same account and have one vote.