I actually did try to add apps to the other preset before pasting.
However once I turned both presets on and proceeded to do the copy & paste, I came across two new problems:
The pasting was buggy and unreliable. Sometimes nothing happened. Sometimes there were duplicates. Sometimes there were errors.
When deleting one of the duplicate apps, I lost all triggers in the remaining one and there was a scary error message. It fixed itself after a BTT restart.
This whole process is so painful and fills me with fear of data loss
I would like to feature suggest a "Merge Presets" button in the preset selector.
Clicking on that button will show a popup that allows you to select the presets to merge as well as the merged preset name.