I use Magpul BUIS and a EOTech XPs2 and have a perfect co-witness. The red dot sits precisely on top of the front post sight when the sights are up.

When you mount the EOTech, or any other optical sight, make a note of which rail number the sight is mounted on, that way you can remove and reinstall it without losing Zero. I switch between my scope and EOTech and never lose Zero on either one. I put the rail number on the bottom of each sight as a reminder.