I don't get why everyone says "Olsen Twins" as if they need both of them. There's only one Michelle. And, as someone noted above, Elizabeth looks enough like them that she could just as easily pass for adult Michelle.
Because they were both Michelle on the original show. I realize that was because of child labor laws, but I can't believe that if they did try to get Michelle into the show they wouldn't extend an offer to both of the twins, since they were both the originators of the role.