Can I add a second Siri Remote? If so what are the steps?

No

Why did you want to use a second remote, you can use a silver remote at the same time as a Siri Remote, it will do most of what the Siri Remote will do except the voice stuff.

Alos, if HDMI-CEC is enabled, you may find that the TV navigation buttons, Enter, and Return buttons control ATV. I use this regularly.