IIS passing Windows Authentication between two sites on same domain / server / app pool

I have two web applications hosted in IIS in the same domain on the same server in the same app domain with the same machine key.

When I use Forms Authentication between the two sites, all works well and I only have to login once and it passes the auth cookie between the sites correctly.

When I try to use Windows Authentication (NTLM) it does not work correctly and I have to login to each site.

Is there any way to get the two sites to pass the login correctly when using Windows Authentication (NTLM) like it does when using Forms Authentication?

<authentication mode="Windows"><forms name=".TESTAUTH" cookieless="AutoDetect" slidingExpiration="true" protection="All" timeout="600" 
        enableCrossAppRedirects="true" domain=".test.com" /></authentication><machineKey validationKey="1234" decryptionKey="1234" validation="SHA1" decryption="AES" />

I posted this same / similar question on StackOverflow but did not receive any responses.

https://stackoverflow.com/questions/44552938/sharepoint-or-iis-in-general-passing-windows-authentication-to-iframe-calling-an