DYI, dual install is not supported for production work.
From the ST8 readme file:
Multiple versions of Solid Edge are now allowed to be installed concurrently for testing core Solid Edge in a non-production environment. (Add-on products, such as Mold Tooling or SE Teamcenter Client (SEEC) or Solid Edge SP will not work.) With ST8, Solid Edge ST4 or later releases may also be installed. The older version(s) must be installed first, and then ST8 must be installed silently. To switch between versions, you have to activate the version you want to use, and this is done with a new program called “SESetActiveVersion.exe”. The program is located on the installation media in the folder \Solid Edge\SptTools\SESetActiveVersion. You can run the program from any location. In the dialog, select the version you want to use then select “Activate”; when the dialog goes away the selected version is ready to be used. If a maintenance pack needs to be applied, be sure to activate the appropriate version of Solid Edge prior to the MP installation.
Note: Multiple version installations are not supported for production use. GTAC will only verify problems using a single version install, as well as Development will only address problems that are reproducible using a single version install. Multiple version installations are to be used only for comparing basic Solid Edge functionality for the purposes of demonstration or pre-upgrade testing. If you need to run multiple versions in a production environment Virtual Machine configurations are the recommended method. When moving to production, it is recommended that you remove all versions, reboot the system, and then install a single product version.