In Palau they only had two tribes so the remaining members of the tribe just did fire and the winner stayed and went to the other tribe.
In Philippines I think it was just a random draw between the remaining two people from the tribe.
But if they ever go down to just 1 person (not that production would ever do that without doing a tribe swap/merge), I'd imagine they just have that person show up the next time and they become a "free agent" where whoever loses the next immunity challenge votes off someone and that other person replaces them on that tribe.