Single Sign-On from Multiple Sites

My organization has separate .net sites for our Intranet and Extranet. Both are secured with SSL and .net forms authentication. We have a separate site for some shared documents and images that both sites need to access. We want to secure that site as well but I'm unable to simultaneously autheticate both Intranet and Extranet users on this 3rd site. It seems that only the default default roleManager is used. The machineKey is set up identically on all 3 sites as are the role names. Is there a way to accomplish this?

Here are the key web.config elements I'm using on the documents and images site:

<roleManager enabled="true" defaultProvider="CDI_RoleManager_Intranet">
<providers>
<clear/>
<add name="CDI_RoleManager_Intranet" type="System.Web.Security.SqlRoleProvider" connectionStringName="CDI_Membership" applicationName="CDI_Membership" />
<add name="CDI_RoleManager_Extranet" type="System.Web.Security.SqlRoleProvider" connectionStringName="CDI_Membership" applicationName="CDI_Extranet" />
</providers>
</roleManager>

<authorization>
<allow roles="User" />
<allow roles="Power User" />
<allow roles="Admin" />
<deny users="*" />
</authorization>

<machineKey
validationKey="{Removed for this post}"
decryptionKey="{Removed for this post}"
decryption="Auto"
validation="SHA1" />