Do I need to own two different Windows 7 licenses to run windows 7 from two different hard drives on the same computer? These two instances cannot run at the same time.

No, should not be a problem.Rich

This is copied from EULA in Windows 7 Ultimate:2. INSTALLATION AND USE RIGHTS.a. One Copy per Computer. You may install one copy of the software on one computer. That computer is the “licensed computer.”I suggest you call 1-800-426-9400 (select option 4), Monday through Friday, 6:00 A.M. to 5:30 P.M. (PST) to speak directly to a Microsoft licensing specialist. It may be changed in different location.