Can you work on a new movie while one is exporting in imovie?

Yes, you could edit another project while sharing out if your computer is powerful enough. Rendering a movie is a very computer intensive process. The more processing power that you use up with additional tasks the slower the rendering process is likely to be. I did a little test by working on another project while sharing out a movie. As I added media, split clips, added audio, added a Ken Burns effect, added a Picture in Picture effect, I noticed that the radial progress circle for the share-out got knocked back when I added the various edits to the new project. Then it would jump forward again. For example, it had reached the half way point but then got knocked back to the one quarter point. So editing was having an impact on the sharing out process. I suspect that it took longer to share out than it otherwise would have. However, after the share out was completed, the movie played fine in QuickTimePlayer. That shows that it can be done.

Still, I would recommend that you let the movie finish sharing out before working on another project-- particularly if the rendering movie is a complex one with a lot of heavy editing. That would avoid any possibility of disrupting the rendering process to the point of causing errors or aborting.

-- Rich

Thank you for the kind sentiments. 😊

— Rich