Only one person per tank. We have two float rooms, so up to two people can come float simultaneously at our spa. Double-wide tanks do exist, but we think that they take away from the whole ‘sensory deprivation’ experience.
There aren’t any latches on the tank doors, and you can get out anytime. That said, the 90-minutes usually goes by waaaay faster than you’d expect (our average customer usually says it felt like 35 – 45 minutes long).