This is exactly what I did to my old tank. Used both as drains, and ran the returns up and over the back.I want to convert a stock overflow box to have an emergency standpipe for piece of mind I plan on using the old return hole as the drain and keeping the factory durso drain. Ile hard plumb the return over the edge of the glass see attached photo for diagram
Not a single drain issue in 11yrs on that tank. Worked great for that setup.