Hi, Cain. I have a few questions that should help us get to the answer.
First, how did you install IC2? Just making sure you didn't download the archive and extract it or something. I'm assuming you installed it via the integrated plugin repository?
Second, what version of IC1 do you have installed? You need at least version 1.8.1.3 (current is 1.8.1.4) installed if you want to switch back and forth between IC1 and IC2. Otherwise you'll get some nasty errors at IDE startup.
Third, can you confirm that when IC2 fails like this, IC1 is completely disabled? This is related to the previous question because IC 1.8.1.3+ will automatically detect both being installed and enabled and help you pick one. If you're on an earlier version of IC1 it's possible you just keep running into the issue of having both enabled at startup.
If those don't get you to the answer, please email me your idea.log (Help>Show Log in Explorer/Finder/Files) and hopefully I can see the issue quickly.
Regards,
Scott Wells