Just to expand on what muzzy996 is saying, the capability of what you are asking for does exist, but depends on whether or not you are rooted.
The solution muzzy996 is for non-rooted devices.
With a rooted device, you can also install the Xposed framework and the Wanam kit for TouchWiz, which will allow you to add any number of non-Samsung-approved apps to run in multiwindow. For floating windows, I use an app called Pen Window Manager that allows me to enable floating window to any app. I suppose Pen Window Manager will also enable multiwindow for non-approved apps as well.