I once thought about doing a double with two girls, i then started watching guys in porn movies try to do it with two girls and the second girl is mostly is flashing her pussy or playing with herself and decided against it, I have done girls in 3 consectuive days and that wore me out as I was very happy though, better bang for your buck? The double I might try is one girl for an hour and then cross to the next bedroom for hour number 2, that double makes more sense to me.