You can have both versions installed on the same computer, and there should be no conflict.
One reason to keep PrE 10 around, at least for a while, is that some users liked some of the Menus and Titles better in that version, than the ones that download for PrE 11.
Also, though a PrE 10 Project should work in PrE 11, it always pays to finish Projects in the version, in which they were started.
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Do you need the hard drive space?
My vote is for leaving Premiere Elements 10 installed for now even though you have just installed Premiere Elements 11. See how you and your computer and your video editing needs fit the Premiere Elements 11 experience. Just use one program at a time. Except for the interfaces, the two versions are quite comparable. If you need to know the differences (in detail) between them, you can browse through my very detailed Premiere Elements 11 First Look thread that I wrote when I was at another forum.
Although it is a Premiere Elements 11 First Look, I frequently make comparisons between 10 and 11 in the thread posts. If you have any questions about what I wrote in that thread, please ask me here at Adobe. Important Premiere Elements 11 information can be found in the Premiere Elements 11 Daily Discoveries thread that I also wrote while I was at that other forum.
When you are comfortable with Premiere Elements 11, then make your decision on whether or not to deactivate Premiere Elements 10 and then uninstall it.
In the beginning you may be in a rush to get a project out and run into a problem executing a Premiere Elements 11 workflow. In that instance, you may benefit from having a backup in the Premiere Elements 10 that you are used to.
Add On...after I posted I saw Hunt's reply...with reference to his remarks about menus...one of the perks of version 11 is that the menus include "free" many if not all of those that were classified as "Plus Members Only" in version 10.