There is no absolute rule here. All come back to us how we had fun with it & create our own version of rule.
Few times i play this fun games version. Normally when we had less player coming or on the last half hour where everyone tired & there still some want to play yet no one want.
Basic rule when we play 2v1, the 1 play single rule & the 2 play double. But when the skill gap are to much, sometimes i would play double rule to & even worst when its totally beginner i would put more handicap as using service area at the front court forbiden to me.
Like few weeks ago, there is only 3 people coming including me. I could enjoy normal single with my close friend (we both more or less on the same boat). But the other one is basically high beginner or early intermediate. To makes him able to had fun (he had drive 1 hour long to come here anyway. Sending him home would be rude & both me & my friend definitely wont last long playing single for 3 rented hour. So little happy games in the middle wont hurt either), we would play little fun rule. I play full single court, while he play half court (we put our bags on both left & right side plus our towel, & etc to mark his area of play). It wont be to much exhausting to him yet everyone could enjoy the games.
Create your own version of rule with your friend & makes agreement where everyone would follow the rule.