Endless won’t let you do that. Such an install is complicated and less common. That is beyond the scope of Endless OS. Maybe someone here could figure out a way to make it happen for you, but it won’t be easy.
Can I get the hard drive to load automatically as a home subfolder location?
As an example:
/home/user_one is on the main 512 GB drive, but then /home/user_one/Documents is on the new drive as a link (the real folder would be /mnt/alt_home/user_one/Documents)…