Can I install MPLAB® IDE v8 and MPLAB X IDE on the same computer?
Yes, both IDEs will coexist on the same PC. They are completely separate programs with their own settings and installation directories. While it is possible for the two IDE's to use the same source files, the output and debugging files are not co-mingled.
MPLAB® IDE v8 and MPLAB X IDE use different USB drivers to communicate with the programmer/debuggers. When switching between the two IDEs, you will need to run the Driver Switcher.