I JUST MANAGED TO GET EL CAPITAN DOWNLOADED ON MY MACBOOK AIR. HOW CAN I OPEN TWO WINDOWS SIMULTANEOUSLY?
HOW CAN I WORK WITH TWO WINDOWS AT THE SAME TIME?
MacBook Air (13-inch, Early 2015), OS X El Capitan (10.11)
If you are referring to Split View, click and hold the green maximize button in an application until it shrinks the window slightly and then drag it to the left or right side. More details and options described in http://osxdaily.com/2015/10/01/use-split-view-mac-os-x/