M.B.
I agree with the weight limits on the bassinet or rolling, PLUS when they start getting up on hands and knees, it is definitely time to move them. Bassinets are not as deep, and I would worry about them managing to get over the side.
Mine usually stayed in the bassinet for a couple of months... mostly for convenience to have them near at hand.
I had kids sharing rooms from birth (or shortly after) on..... but there wasn't a 6 year difference. They each learn to sleep through the other noises from an early start this way.