Definately, just make sure the two eclipse and MyEclipse installations leave eachother alone, a la:
C:\Java\Eclipse2.1.2
C:\Java\MyEclipse2.7
C:\Java\Eclipse3
C:\Java\MyEclipse3.7
So unzip botht he versions of Elcipse you want to use in differently named dirs. Then run the 2.7RC2 installer, and point to your 2.1.2 installation. Now run the 3.7RC2 installer and point to the 3.0M6 installation. You should be ready to go.
NOTE: After installing a fresh copy of Eclipse, be sure to run it atleast once so it can initialize before installing/running MyEclipse.
C:\Eclipse2
C:\Eclipse3