Configure ADFS Web application proxy for two domains

I am setting up an ADFS server to manage authentication to websites that are going to be accessed on a internal domain and a external domain (The internal domain is not the same as the external domain). These websites are hosted on a separate web server from ADFS. I followed these two guides and have doubled checked everything, and I believe everything is set up correctly.

https://www.virtuallyboring.com/how-to-setup-microsoft-active-directory-federation-services-adfs/
and
http://social.technet.microsoft.com/wiki/contents/articles/34162.ad-fs-4-0-discover-setup-and-publish-application-part1.aspx

The problem I am having is that I need auth.domain.com to work both internally and externally. Externally I need auth.domain.com to point to the proxy, but internally auth.domain.com needs to point to the internal ADFS server. This can be done by making changes the DNS and Firewall. By doing this I am having to maintain two different sign-in pages because one is shown if the site is accessed on the network and the other is shown if the site is accessed off the network.

How can I go to auth.domain.com internally and be pointed to the ADFS server, and go to auth.domain.com externally and be passed through the proxy to internal ADFS server?