Can i place a [locked] sign above a double-chest and the lock applies to both chests (as it's a double-wide), or do i need to place 2x locked signs? (tried searching forums but couldn't find anything which mentioned this, only single chests)
Yup! A [ LOCK ] sign applies to an entire double chest. You could even place a second [ LOCK ] sign above the second part of the chest, to give a 4th and 5th person access to it.