So Sleep Clause is a very old rule that applies to most of Smogon's single metagames. Basically, it means that you can only attempt to put one Pokemon to sleep - it won't prevent you from putting an opponent's Pokemon to sleep if they have another Pokemon who used Rest, it only covers sleep inflicted by the opponent. It was removed from Doubles OU in a previous generation and has stayed such through the current meta.
In short, in DOU you're fine putting multiple Pokemon to sleep. If you're unsure about what clauses apply in a game look at the top of the log next to the battle screen - there'll be a list of clauses in effect for that format there. The person who told you that sleep clause is a thing in Doubles OU was most likely uninformed, because Sleep Clause is in effect for most metagames, but if the simulator allows it then you're fine.