- Go to System Preferences -> Users & Groups
- Click on Login Options
- Unlock / Authenticate
- Turn on Show Fast User Switching Menu as
- I normally recommend Account name
- Close System Preferences
This will enable Fast User Switching. The current user is still logged in, so you have both users logged in at the same time. If you are done, please remember to log out of the user instead of Fast User Switching back.
If you fast user switch back to the original user, you haven't logged the other user out, so you are consuming more RAM, and resources than you need to.