Two best friends with Down syndrome who caused a sensation four years ago when they created their own gory zombie movie are back, this time in a documentary championed by a Hollywood luminary. ‘Sam & Mattie Make a Zombie Movie,’ released on Tuesday on Apple TV, follows Sam Suchmann and Mattie Zufelt on their tenacious 10-year quest to storyboard, script, produce, cast and star in 2016’s ‘Spring Break Zombie Massacre,’ a comedy slasher movie complete with severed heads and spurting arteries. The original movie earned national attention and an appearance on Conan O’Brien’s show for the two Rhode Island natives.