I know some cosleeping moms put a parent between the baby and the toddler to keep them separated. Another option is to get the two year old into his own bed in your room at least to start with. The solution depends on whether you hope to continue cosleeping with your DS. Many (though not all) kids sleep longer stretches in their own space and even though moving him would require you to get up when he wakes, it may be worth it. Good luck! If you do decide to make a transition, I recommend sooner than later so that the new arrangements are very well established by the time baby comes. We got DD a double so that on rough nights I could bring baby in and the three of us could cosleep. Ideally DD will sleep through the night and I can tend to baby but I can handle the one wakeup if needed.
I am right there with you: six weeks along with our second, and our first is also breastfeeding and co-sleeping. We are considering night weaning, but there is always some reason (sickness, travel, etc.) why we keep having to put it off. I am eager to hear what the others have to say!
I have never co-slept with multiple children, and I am not doing it now (we have the baby in a crib in our room), but I would recommend getting the 2-year-old in her own bed asap. That might also cut down on her night wakings for milk.
Congratulations! I BFd DD until she was 22 mos old. DS was born when DD was 25 mos old. I co-slept with both of them (still do), and as a previous poster said, we put a parent in between the two of them - that parent was me. We have a king sized bed but had bolsters on both sides, a toddler and a baby, so there wasn't room for both me and DH.
Now that they're older (6 and 8), we all 4 sleep in the same bed (no more bolsters - thankfully!). Sometimes DH snores really loudly to the point that he wakes me up. When that happens he ends up in another bedroom.
ETA: Regarding the baby waking the toddler in the middle of the night... I don't remember this happening much, if at all. DS was a really great night sleeper. When he did wake, he'd nurse right away and be back asleep without waking DD.