Well it was a while ago with DS and we're not there yet with DD, but we did short am, long pm naps with DS so a slightly different scenario. I know everyone I've heard from has had a messy experience of the 2-1 though, just to warn you, but this is your third, right(?), so you'll probably already know that
So we hung onto the morning (UT) CN, capping it shorter when needed, until he started refusing it at home, but continued to take it with his CM, up until the spring daylight savings, which fell in a school holiday so we could be more,flexible, and worked conveniently in our favour as he could then naturally wake later to have his one nap only a little earlier than previously (but an A time of 5h) - which fitted in well with the CM. Not sure if that makes sense, and of course would only work so neatly with the spring daylight savings! Anyway, the one nap was of variable length to start with, he might wake after 1.5h one day but resettle; another day he couldn't resettle. This seems to be a common experience around here before it finally lengthens and stabilises. Some LOs will do 3h straight! Mine did 2.5h at first, but after a while I had to start capping it as we were getting EWs and he needed to be a bit more tired at BT to do a decent night.