Jemima/Sillabub is usually singing solos at the beginning of Memory part I, during The Moments of Happiness and Memory part II (see in the Quotes page)
 
During the show, you can usually see her hanging around with the other kittens or sitting at the back with Old Deuteronomy, and during all dancing parts
 
Some productions have both Jemima and Sillabub. In that case, Sillabub is the one who sings the solos and Jemima is a chorus kitten.
In Sweden (around 1988) they had both Jemima and Sillabub. I don't know what were their parts. But none of them sang Memory, they had a "young Grizabella" for this part.
In Japan, they have both too. Sillabub is a sort of Victoria / London Jemima mix. She's the one who sings Moments of Happiness, Memory, and is the one who touches Grizabella at the end of Memory. In that show, she's the one who represents the innocence. Jemima there is an older kitten, maybe even a young queen.