I don't know what is causing the issue, but if you don't actually use the .xmp file for anything you can just turn off the .xmp option in preferences if you like. This way the issue will go away. If you go to "edit>preferences then select the "media", you can then uncheck the "write xmp ID files on import" option and also for disable the other XMP option right next to it. Then it will stop doing this at least, sorry I don't have a answer for what is causing the issue though, although I would like to attempt to figure one out.
What version of Premiere cs6 are you using?
Also is it only happening with the same exact two files every time? (Basically I'm wondering do other files XMP's cause issues like this specific one does?)
If it is only this one specific file that is causing the problem I'd probably say it's safe to guess that something about that file is just corrupted, and that is probably what is causing the problem.
The other possible work-around though is you can use a .MTS clip merger program that will join the files together rather than using Premiere media browser to join the two seperate clips and see if the .xmp issue stops.
Because cs6 currently has known bugs with AVCHD spanned media. The clip merger works really quickly too, because it doesn't actually encode the media it only merges them. It's also free and simple to use which is always a obvious plus side as well. If you'd like to try it here is the link where you can get it from.
Following your suggestion, I disabled the "write xmp ID files on import" option and also disabled the other XMP option right next to it.
The XMP issue occurs in other projects.
I am unclear from which file the Premiere media browser obtains information about which MTS files make up a spanned media i.e. a shot that is say one continuous hour.
If the problem doesn't go away now that you turned off xmp writing I'd recommend trying to use this clip merge program. This way you won't have to use the media browser to join anything anymore, it also makes media management much easier.
If I merge the MTS files for a continuous talk, does MTSMerger.exe change the CPI, MTS, MPL, index.bdmv, and MovieObject.bdmv files to reflect the change? Will I lose metadata about time/date codes, etc.? Thanks
It will not update any of that stuff, and actually you'll lose your metadata. Because MTSMerger creates a file that is free from the folder structure. So if you join stuff using MTSMerger in my personal experince you no longer need the folder structure, because you don't need to join any of the clips together in Premiere because everything is already merged together. It makes media management much easier, however you do indeed lose your metadata. As far as I'm aware there isn't anyway around that.
Did you delete all the .XMPs you have after turning off the two options though?
Yes, I deleted all the .XMP files since they caused Premiere to not load the project.