Having both installed won't cause any conflicts that I know of. When X3 first came out there were same serious crashing issues so I kept X2 installed on my desktop. Now that the issues have since been resolved I still haven't uninstalled X2 and X3 runs just fine.
Once you're comfortable with x3 you could delete it if you need the space. I have 2 versions running (my older one is PSP7 from years ago) because I use one for photos and one for creating graphics.
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