One thing you can take a look at...

Properties telling your phone to "wireless sync" your calendars. If you've got both set to wirelessly sync... thats what they'll do.. if you uncheck one of them, then go through DM and go to backup\restore > advanced... you can actually Clear out everything in your calendar (wont affect information stored on google or yahoo..) and then reload only the 1 calendar you want on your phone!