For the backend you can only have one membership provider. You can have multiple providers for the frontend of your website. Generally you can extend the backend login and allow some users to access it even you use different provider for authentication.
You need to check whether the user that is trying to login belongs to the other membership provider and create the same user in the current backend membership provider.
MembershipProvider prov = Membership.Providers[
MembershipUserCollection uColl = prov.GetAllUsers(0, 2, tRow);
(uColl.Count > 0)
(muser.UserName == UserNameTextBox)
// proceed with creating this user in a temp role
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