I'm not sure... I do believe it's gaps and things that you need to be careful of. I don't know what the gap with a bedrail would be like (DD's bedrails aren't up any more.. so can't remember).

We coslept tho by having DD in an armsreach cosleeper cot, and then DS was welcome to come into the middle of our bed if he woke up. DD was in the armsreach for longer than usually recommended as she was still wrapped for a long time and didn't move around. Whereas they say once they're rolling and on the move not to use it cos they can get out and fall off your bed. By the time she outgrew the cot and came between us (around 8 months i think??) then if DS came he just squeezed onto the edge on my side. Not comfortable for all night sleeping I can tell you... so this was when I started taking him back to his own bed if it was too early in the night. If it's around 5.30 am or something he can stay haha. And it's still this way at the moment! DD is between us most nights once she comes in, and DS come in early morning. Squishy, but lovely

But yeah.. can't really help with the bedrail sorry. I'd probably be inclined to put bub in the middle and have the toddler on the outside if we were doing it without the cosleeper cot.