There's plenty of those around!
Going back to Revolution 1, Francie Schwartz has claimed in the past that she and George did the "shoo-be-do-wop" vocals. Could it be those two doing the 'mama, dada' ones too? (Doug Sulpy reckons it's John and Yoko, for what it's worth.) I know they and the "shoo-be-do"s appear at the same time in the mix, but one or both could be an overdub.
Lewisohn didn't mention Schwartz in the Recording Sessions book, but in interviews she's said that she participated at the 4 June 68 session. Elsewhere I've read that she's a bit of a fantasist and is prone to talking self-regarding nonsense, so I'm not sure where the truth lies. Any thoughts (with regard to this recording, in particular)?