So I got my hands on a Pi400 that I plan on running Octoprint on. My question is, can I multi-task on it so I can have it run Octoprint but also have it run a "desktop" so I can display something on a screen for when I shoot videos or livestream?
The 3D printing process is a semi-real time process (the firmware in the printer handles the actual real-time part). If the serial pipeline isn't kept full, then the printed object will have zits and blobs on it (or worse).
So to answer your question... Yes, you can do other stuff but you run the risk of ruining your printed objects. I have in the past installed the "desktop" on my OctoPi system attached to my printer but I never tried to use it while I was actively printing. Currently, I don't have the desktop installed on the system attached to the 3D printer.