Can I install multiple MPLAB® X and MPLAB 8 IDE versions on the same machine?
Yes, you can install different versions of the MPLAB® X IDE on the same PC, or have both MPLAB 8 and MPLAB X on the same computer. In this situation, the only restriction is you must use the MPLAB Driver Switcher in order to switch from one USB development tool to another. This utility is automatically installed with MPLAB X IDE. The reason for this is that each IDE uses a different set of drivers for the USB based development tools.
Go to "Swap USB Drivers Between MPLAB 8 and MPLAB X" for more details.