The line outs from the back of the Focusrite should go to the headphone amp.
That has two benefits;
The headphone amp gets the expected signal (with separate volume controls on the headphone amp)
The headphone outputs on the Focusrite are still available

So you've gained headphone outputs (on the Focusrite) and retained signal quality to the headphone amp.
We've been running that way for years with an 8 Out headphone amp.
Using a headphone out to feed a headphone amp would degrade the signal IMO

I do not work here, but the benefits are still awesome
Make your sound your own!