How to combine two vidoes side by side

I’ll use two images (could be video) of the same resolution of 1920x1080.

First step is to make a custom Video Mode.
Resolution 3840x1080 (set Aspect Ratio 3840:1080)
Setting this custom video mode will not “bake” in black bars. If this is for YouTube, YouTube will accept any resolution size you throw at it and there will not be any black bars.
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On each clip you’ll have to use Size, Position & Rotate filter, manually typing in 1920 for the first box for Size (will automatically update the second box).
shotcut_2020-10-25_18-01-38

Use the grid tool for alignment. I used 4x4 with snapping on.
To see what it looks like just click on Output to not view the filter’s VUI box. Make sure you click on a clip again for adjustment thus not putting a filter on Output by mistake.

Here is a frame exported from Shotcut.
To export frame: File - Export Frame