Hi,
You need to use semicolons to separate roles in this property. Example:
DisplayToRoles="SilverPartners;CMSEditor"
You can alo check current user role and according this role set visibility of your web part/control:
bool CMS.CMSHelper.CMSContext.CurrentUser.IsInRole(string roleName, string SiteName)
Best Regards,
Miroslav Remias.