If Account.master is a child page of User.master then you can find controls of User.master page by doing something like following-
ContentPlaceHolder cp = this.Master.FindControl("ContentPlaceHolderIdOfUserMaster") as ContentPlaceHolder;
DropDownList drp = cp.FindControl("drpYourDrpId") as DropDownList ;
Else, you can use other state management techniques like
Session,
Cookies etc. to pass value from one independent master page to another independent master page.
For this purpose, check these links-
Transferring page values to another page[
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How to: Pass Values Between ASP.NET Web Forms Pages
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Hope, it helps :)