For one given version of SketchUp (2017, 2018, etc.), you cannot use the Euclid and OpenStudio plugins at the same time -- you can only use one. If you want to switch between Euclid and OpenStudio using one account on your PC, then you will need to disable what you don't want to use and enable what you do want to use.
Refer to this post for more details, as well as how to enable and disable the two plugins.
EDIT
After reading the question again, I think that the root cause of your issue is the installation process. When you install a SketchUp plug-in for SketchUp 2014 and onwards, the files are automatically saved in C:\Users\"User Name"\AppData\Roaming\SketchUp\"SketchUp Version"\SketchUp\Plugins. So, if you have two accounts, those should have a separate user name. This means that in order for both Euclid and OpenStudio plugins to work for both of your accounts, you will need to install BOTH plugins while logged in to each account. Then, you should be able to enable and disable plugins from my original post above.