It's nice to work with a couple other people around your level so that you can watch each other and learn from each other, but not so many in your workout group that you get bored having to wait your turn. It's not just less efiicient, it's less fun, better to go go go, allez allez allez lol. Ya old song I know lol. Anyways, the best workout I say is one that is flexible and if you want to practice more one day you can, and if you want to practice less one day you can, etc. Double workouts can be good in that you can have a break between workouts, it's not straight through, but it's nice if it's tailored to you so that it will be beneficial and you can have options.