RESOLVED!!!
I'm running Windows 7 Pro X64, and when I click to run the other version, it gives me an error.
After Clicking that a couple of times, it says that processes are interfering with inventor and must close to continue, yet it gives access denied even though I am on an admin account.
After thinking about what you said, I realized that Inventor was not in fact the problem, it was user account control and windows itself.
FIX for running more than one version of Inventor on Windows Vista (If Applicable) and Windows 7:
Fix 1 - Requires UAC consent
Step 1: Create a shortcut for Both Versions of Inventor you are planning on running.
Step 2: Right Click on the shortcut and select Properties
Step 3: Under the compatibility tab, Check the check box that says "Run this Program as an Administrator"
Step 4: Click Ok and you are done! - NOTE: User Account Control will prompt for consent or credentials if you have it turned on. Click Yes when UAC Prompts for inventor.exe
Fix 2 - Using Task Scheduler - Recommended for Experienced Users only
- NOTE - THIS FIX will NOT Trigger UAC,
Making this a good work around for people using non admin accounts
See the Microsoft Technet Article
Here
Follow the Steps and Create 2 Tasks.
1 for each Version you plan on running.