You can leave Premiere Elements 11 installed and activated (but not opened) when you install and activate Premiere Elements 12 on the same computer.
Just use one program at a time.
Just in case
a. Be sure to have latest version of QuickTime installed on same computer.
b. After you install and activate version 12, update 12 to 12.1 Update using an opened 12's project's Help Menu/Update.
Both 11 and 12/12.1 should co-exist nicely.
Please let us know the outcome.
(From personal experience, Premiere Elements 12 can be a nuisance getting beyond the Sign In whether or not 11 is involved. But,
let us take one step at a time.)