I seem to remember when the show first started that they hid the fact Michelle was played by twins and they credited her "Mary Kate Ashley Olsen".

I don't know exactly when they changed it but when did they credit both of them with the "&"?

The 8th (and final) season was the only season where they were credited as 'Mary Kate & Ashley Olsen'. Most people knew years before that Michelle was played by twins, as the girls made numerous appearances on talk shows, plus they had their own production company.

I don't think they ever hid it. I knew that they were played by twins right from the first episode.

I don't know how true it is, but I have heard that at first the show didn't want to reveal that 'Michelle' was two people.

I'm curious to know if this was the case and why.

I can't remember when they changed it to Mary-Kate and Ashley Olsen on the credits, but at first they put Mary-Kate Ashley Olsen because they didn't want the audience to know that they were twins yet. Also, in the last episode when Michelle had abnesia(i'm not sure how to spell that) she got her memory back by going into her twin(or something like that) because the other twin was her memory.:)

I read that as they got older, the producers thought about only using one of the girls and John Stamos convinced them to use both Mary Kate and Ashley for the remainder of the series.