Our son's shared a room from the start but Yasin was 16 months old when Imran was born not 3.5 so he couldn't possibly get into the cot.
Also when I say shared I mean it very loosely because Imran mostly slept in the hammock down stairs or in my bed so mainly it was Yasin's room and Imran's clothes and so on where in there. Over time Imran slept in there more often and now they go to sleep together (they co-sleep in a double bed) most nights although usually at least one of them will migrate to my bed overnight.
I would think that if I were in your position I would move them in together very gradually rather than all at once.