Multiple membership providers into users and roles

In my web.config, I have multiple membership providers from 3 different domains.  The root domain and 2 child domains.  I need to be able to add users from the child domains into my roles, but my it seems to only let me add users from the domain that is listed as the "default provider" in the web.config.  How can I do this?  Thanks

?
Private Sub BindUsersToUsersList()? 
Dim users As MembershipUserCollection = Membership.GetAllUsers()? 
UserList.DataSource = users?        UserList.DataBind()
End Sub  

<membership defaultProvider="MyADMembershipProvider" >
?   <providers>?     
<add connectionStringName="ADConnectionString" ...