Sure. Install only one Manjaro edition. Then it simply becomes a choice of which edition you need/prefer.
The easiest way for you to “start over” is likely to boot the Manjaro ISO/USB of your choice (let’s say, KDE) and open GParted from the Live environment’s menu – delete both Manjaro partitions (/ from both XFCE and KDE) – Reboot and boot the ISO/USB again – Install your preferred Manjaro edition.
You have for all intents and purposes done this twice already, with success (despite the XFCE booting issue). I see no reason why choosing only one instance of Manjaro (instead of two) will be any problem for you.
Every Linux distribution has it’s own idiosyncrasies; it’s own learning curve. I can make no guarantees that you wouldn’t have encountered issues with what you suggest; albeit, not the same issues.
This is generally referred to as multi-booting.
Multi-booting has become fairly popular over the years, especially with Linux and Windows. I have personally done this myself for the better part of twenty years, currently with Debian, Manjaro, MacOS, FreeBSD and Windows.
The ongoing success of my multi-boot scenario is mainly attributed to having each OS on it’s own disk (not partition).
Regardless of the popularity of multi-booting one important fact should always be remembered – no OS is designed with multi-booting in mind – there is always the potential for something to not go as planned (as you already found).
I wrote a general guide for Linux/Windows that may be of interest if you are curious enough:
Also another guide about Ventoy; an alternative method to boot several ISO’s from USB:
I would be remiss if I didn’t also address your obvious level of Linux knowledge, generally. It may be that a rolling release distribution such as Manjaro is not for you.
It’s possible that you might be more comfortable with another point release distribution such as Debian, for example. Only you can decide.