From what I understood, if it's two leaving a week (one Saturday and one Sunday), it's whoever has the least votes each night. If it's four in a week (which will have to happen at least once), so two each night - then it'll be one from each category, or else a judge could conceivably lose two acts in one night.
Which is all kinds of unfair. If there's two crap acts in your category, they deserve to go. This is part of the reason why The Voice is so ****, because they insist[ed] (idk if they still do) on reducing the teams at an equal rate

the lives are the worst part of that show and they're literally copying their format...tragic.