Exactly what the title says. I have a run that is a PB in 2 different categories, but the video proof is super long and it already took about half a day to upload it once. While the other video would be about half as long, it would still take a long time. Would it be okay if I used the same video for this other category?

a lot of people do that as long as the video is a run of both categories (for example a run with/without items, if you do a run without items it is technically a run with items as well) However some moderators prefer if you submit just the part of the run that is applicable, but there is nothing against it.

If you're using YouTube, it's generally helpful to provide a link with a timestamp. Just go to the start of the run, right click on the video and select "Copy video URL at current time".

If you're not using YT, you could just leave a comment saying when the run starts.