We coslept until we nightweaned around 18 mo, then I decided to try putting them in their own room (have always nursed them to sleep in the family room & then carried them upstairs, so figured could carry them to their room as easily as ours) & it worked fine so we went with it. It sounds like you're putting them down awake, which we haven't tried yet, so not any real help with that, but I have found that anytime my girls start taking forever to nurse down, it's time to tweak the nap schedule. I don't remember exactly, but sometime relatively soon after 12 mo (I'd say within a couple months), I was having a horrible time nursing them down, took forever, really wiggly, etc.) I cut out their morning nap & moved their afternoon nap earlier and they went back to nursing right down at night. Since then we've had a couple times when they start taking longer to nurse down & I can usually "solve" it by making sure their naps are never longer than x amount of time (current limit is 2 hrs, but I think we probably need to scale that back again, honestly I think we're within a few months of getting rid of naps all together) &/or never letting them nap after a certain time, even if it means a short/no nap that day (currently 4:00, but even that is late, if they nap that late I can count on them going to sleep later than usual that night).
So, I'd start by looking at total sleep time & tweaking naps as needed to help w/ nighttime sleep.
My only other advice is, other than when the girls were itty bitty, they've never slept next to each other, always on opposite sides of me (well, for most of our cosleeping time, dh spent at least part of the night sleeping in the recliner portion of our sofa holding one baby (not always the same one) so then I just had one baby in bed with me, but if we were all in the same bed we had them on either side of me. When we nightweaned (so there was no longer a need to have them on either side of me) we started putting them next to each other but put a pillow between so they wouldn't roll on top of each other. They're both very restless sleepers so I can't see how they could help but wake each other up if they were right next to each other. So maybe look into that possibility as well.