Very much depends on the child(ren), imo.
I waited until ds had literally outgrown his cot, as I thought 'if it ain't broke, don't fix it'. So, he's only been in the big boy bed for about 3 months now.
Would going into big beds disturb their sleep at all? Would they wake each other up? Wonder round their room at night? If not, then go for it, but if you are worried, then I would leave it for a bit. I don't think their are any 'rules' about when they 'should' be out of the cot (just before they go to university
)