Michael,
You can install multiple versions of PADS on the same machine but you cannot run multiple versions at the same time. You install both versions into the same root folder (e.g.: C:\MentorGraphics) and then use a configurator tool to switch between them. This is documented in "PADS Installation Instructions" and the "Managing Mentor Graphics Systems Software" manuals.
https://supportnet.mentor.com/docs/201206070/release_docs/PADS_install.pdf
https://supportnet.mentor.com/docs/201206070/release_docs/manage_sw_pads.pdf
Guy