There's a similar thread - here.
Though I don't think you can make 2 reservations, it should reserve whoever picks one first.

Was 6pm your local time? Because it's in local time and not server time.
The 20 minutes thing, as far as I know, is the time you have to get in, so 6:20pm would be the last minute for you two to get inside.

There's also one thread in 'Accepted Bugs' subforum. :/