With my twins, I had a part-time cosleeping two year old at the same time. We drifted through a whole bunch of different arrangements, during that first year, as everybody's needs changed. My twins were never ever happy sleeping next to each other-- DS was and still is an extremely light sleeper, and he couldn't sleep worth a fart with his twin next to him. (Sorry for the strong language-- I still sometimes wonder how we all got through that phase!

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We started with the queen-size with a side-car crib, and a mattress on the floor for DD1's occasional cosleeping. I mostly didn't use the sidecar at first-- I kept my pump, and extra diapering stuff, and snacks and drinks for me there. I actually had a minifridge under it for awhile...

The twins slept on either side of me, with one twin between me and DH, and one on the crib side. As each twin woke, I rolled to the side to nurse. If both woke together, I'd have DH take one downstairs to walk around until I got the other back to sleep. I can't tandem lying down, and if I tandemed sitting up, when I tried to put them down afterwards, DS would wake up screaming. Sometimes in the first few hours of the night, I'd go sleep with DD1, because the twins would give us a longer stretch if I wasn't there-- like three hours as compared to 45 minutes.
After a few months, we went through a phase where DH and DD2 slept downstairs on the guest bed, and DS and I had the bed. DD2 would sleep longer that way, and when she woke, I would go downstairs and nurse her back to sleep, and then go back to DS. At that point, DD1 was in her own bed again. DS woke very frequently with me next to him, but he was having weight-gain issues, so we didn't want to encourage him to sleep longer stretches. DD2 on the other hand was a chunker, and could clearly manage three or four hours without nursing with just the barest bit of encouragement.
Eventually, when they started crawling, we put our bed on the floor, and the crib across the room, and I started teaching DD2 to spend part of the night in there. I gave DS the mattress on the floor, and I used to get up during the night and lie there and nurse him, and then when he was conked out if I was still awake I'd go back to our bed and sleep with DH. When DD2 woke I'd bring her in with me, and then just bed-hop all night.
That was the arrangement we had when they hit toddlerhood.
I think my point is that with twins, you gotta be flexible, and do whatever works to get each person in the house the maximum amount of sleep.
I loved the roll-back-and-forth stage-- that was my favorite part, and if DS hadn't been such a difficult sleeper, we might have gone on with that way for a much longer time.