There are numerous reasons to have multiple installs of SOLIDWORKS on the same machine, but the most common reason is for working with outside contractors or vendors on a different year version since SOLIDWORKS files are generally not backwards compatible in a write-able format.
If you need to have multiple year versions of SOLIDWORKS installed on your computer, it is best to start with a clean slate, by performing a clean uninstall of all instances of SOLIDWORKS. This is not required, but it is recommended.
Uninstall Best Practices
If you want to uninstall, follow the uninstall guide here for each version of SOLIDWORKS installed: https://support.hawkridgesys.com/hc/en-us/articles/360033746831-2020-Uninstall-Removal-Guide.
Installation Best Practices
After that is complete, it is recommended to start your installs from the oldest version of SOLIDWORKS first. Double-click on Setup.exe from your install files to begin the install.
The SOLIDWORKS Installation Manager will ask you which type of installation you would like to perform. Choose Individual (on this computer) and select Next.
If you have a previous version of SOLIDWORKS installed on this computer, you will be presented with the options to create a new installation or to upgrade your existing installation. Select Create a new installation of SOLIDWORKS 20XX.
At the Summary stage we will need to change a couple locations.
Select the blue text "CHANGE" next to Installation Location, then add the year of the SOLIDWORKS version that you're installing to the folder path. This helps us to identify which folder is for which version of SOLIDWORKS. If we were installing SOLIDWORKS 2018, for example, the Installation Location would look like C:\Program Files\SOLIDWORKS Corp 2018
Now we can select Back to Summary and click the blue text "CHANGE" next to Toolbox/Hole Wizard Options. We will now select "Create a new <your year version of SOLIDWORKS> Toolbox, and then select Browse.
Here we will add the year version to the folder name, to reflect the version we're installing. For SOLIDWORKS 2018 that would look like C:\SOLIDWORKS Data 2018, for example.
This helps us to identify what year version this Toolbox belongs to, for future upgrades and maintenance.
Graphics Card Drivers Best Practices
Certified graphics card drivers for SOLIDWORKS are set by the year version. When you have multiple year versions installed, it is recommended to either run the most current certified graphics card driver for the newest version of SOLIDWORKS, or for the version of SOLIDWORKS you will be using most often.
SOLIDWORKS may have a certified graphics card driver for a specific year version, but that does not necessarily mean it will also be certified for an older or newer year version of SOLIDWORKS.
In the event that you need to switch back to a previous graphics card driver, you can go to Device Manager>Expand the section Display Adapters>Right click on your graphics card>Select Properties>Select the Driver tab>Select Roll Back Driver.
Other SOLIDWORKS Tools Considerations
If you're running SOLIDWORKS PDM you will want to ensure that it is the same year or higher than the highest version of SOLIDWORKS, as putting SOLIDWORKS files into the Vault that are newer than the Vault can cause file corruption.
SOLIDWORKS Electrical Schematic can only have one year version installed. If you're going to be using SOLIDWORKS Electrical 3D Professional with Routing, you will need to choose one year version to use for Electrical 3D design and install Electrical Schematic at that year version.