Check your laws up there. In NC, you can have the total amount of skiers/riders as the ski allows alone. A GTI is a three person ski, so you can have a total of three people in whatever combo.. And here, if you have mirrors, you don't need a spotter. So I can pull 2 in a tube with my GTX.
To answer the question,
We use a two person WOW tube from Sam's Club. One you sit in with a back rest and a place for them to sit. You can also pull it backwards which the kids like too. We also have a flat one, but the kids don't like it as much. Also, the wow tube is easier to get on plane. The flat one tries to go down first. As far as ropes go, we just use a cheapie basic rope.