You can install many versions of RoboHelp but you can only open a specific project in one version. Once upgraded, you should not go back to an earlier version as it risks breaking the code.
I use several versions to help with forum questions that can be about a specific version. I add the versions to my Send To menu and then right click a project to open it in the desired version.

If you have a reason to keep both, that will work but you have to organise your projects to know which need which version. If you only have one project, there is no point in having multiple versions of RoboHelp.
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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.
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