Yes. Installing Paratext 9 does not remove Paratext 8 from your computer. The two versions use the same project folder, and there is no need to migrate projects like there was when moving from Paratext 7 to Paratext 8. Team members can share projects together even while some are on Paratext 8 and some on Paratext 9.
We would advise caution in opening both versions at the same time. Both versions read and write the same files in each project. If you make changes in one version, then changes in the same project in the other version, some of your changes might not be saved. You will get a warning message if you open one while the other is already open. We would advise you to heed the warning and close one before you open the other.
If after installing Paratext 9 you wish to remove Paratext 8, you can uninstall it from the Windows Settings App, (the gear in the start menu > Apps > find Paratext 8 in the list and click Uninstall) or from the Control Panel. (Programs and Features > Find Paratext 8 in the list, right click and choose Uninstall).