How to quickly switch between two apps on one half of the split-screen view? I'm in this scenario a lot:
I'm using split screen opened to App1 taking the first half of the screen and App2 taking the second half of the screen. I frequently need to switch between App2 and App3 on the second half of the screen while keeping App1 on the first half. One way to switch between them is to tab the recent apps button then drag the app window to the second half of the screen. Unfortunately this process takes from 5 to 15 seconds each time I want to switch between App2 and App3, which is very exhaustive as I need to switch between them in a very frequent manner (About 3 to 10 times within 5 minutes).

How could I switch between App2 and App3 on the second half of the screen quickly just by some gesture and the apps get instantly switched without the need to drag or re-position the apps windows in order to re-fit the split screen each time?
A gesture example could be: swiping from the buttom edge of the screen to upwards on the second half of the screen (if screen is in horizontal mode).

Important clarification:
I'm asking about switching apps while being in multi-app split screen view. I'm not asking about single app view.
One example of apps which works very well for the single app view is EAS App Switcher. What I need is that when I switch to another app it's opened in the second half of the splitted screen (not the full screen which destroys the split screen view). 
Let me clarify more. I have App1 and App2 active (in split screen mode). EAS will do the job if it replaces App2's window with App3's while keeping App1's as is. What it does is that it opens App3 on full screen (single app view) destroying the split-screen view and App1 becomes in the background instead of keeping App1 active and next to it is App3 on the other screen half.
It would be very helpful if exist some app or launcher that achieves this.
I'm on Android 6.0.1 on Samsung Note 4 with root access.
Tested on Oreo 8.0. lt works as described without destroying split view

Try EAS App Switcher
it creates a floating icon that can be hidden as well and find it particularly good in switching to previous to previous app , that is juggling with three apps ; 

In Settings Single tap on second icon &rarr; switch to second last app does this

It prompts to you to change the home which makes it faster. It doesn't support gesture navigations that you are looking for but offers some like fast pull to display notifications etc