I'd say start with talking to your ds1 about it, telling him where you expect him to stay in the bed, ask him how he feels about it, etc. Make sure you've got enough room in there for the 3 of you and then figure out how you'll put them to sleep. In my situation, I used to nurse ds2 to sleep in the living room (he would just play around if I tried nursing him in the bed) and then transfer him to the bed. Then I'd take ds1 in and lay down with him until he fell asleep. As ds1 has gotten older I only have to lay in there with him for a few minutes and read quietly, then put on some quiet music and leave and he falls asleep by himself.
Now my youngest doesn't nurse any more so I lay down with him first and I listen to an audio book so I don't go insane waiting for him to stop squirming and pass out.
We haven't had problems with the boys waking each other as long as an adult is between them. Sometimes when dh and I come to bed the boys are snuggled up together and they generally stay asleep for that, too.