Nesting authenication elements for two directories

I am developing a web site where there are two groups of users: sponsors and members. I would like each group member to login based on the group they belong to, but through a different login page. It turns out that I cannot add two separate 'loginUrl' elements to my web.config file, so how would I workaround this? 
I wanted to use: loginUrl="Members_Login.aspx" and loginUrl="Sponsor_Login.aspx"...
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