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.

The problem I am having is that I need to work both internally and externally. Externally I need to point to the proxy, but internally 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 internally and be pointed to the ADFS server, and go to externally and be passed through the proxy to internal ADFS server?