I found myself in a situation in which it was better to uninstall X1 first.
I had installed X1 on a non-C: partition in a folder called X1-something-or-other.
When I then installed X2, a lot of X2 stuff ended up inside the X1 folder, which didn't make much sense.
So I wiped everything clean including as much in the registry as I could find, and then installed just X2.
I have no reason to use X1 anyway and it just took up space on my SSD.