I notice that some of the X2 and X1 vstplugin files differ (e.g., BitMeter.dll, Boost11_64.dll, etc.). Should we allow X2 to overwrite the X1 versions? Or keep them separate?

The following plugins differ:

BitMeter
Boost 11
Channel Tools
Linear Compressor
Linear EQ
Percussion Strip
Session Drummer 3
TransientShaper
Tube Leveler
Vocal Strip

Did anyone allow X2 to overwrite the X1 VST Plugins?

If so, can X1 use the updated versions?

After installing X2 alongside X1, I allowed it to overwrite those plug-ins.
Ive had a few crashes in X2 so I had to return to using X1 for some time based projects and Ive felt no ill affects after allowing them to overwrite the vst's.

Hope this helps
Regards, Lee

Sure, well-written VSTs do not depend on the DAW version.