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.