So what's the difference between the two?
Sometimes, the two words are used interchangeably, so quickly note the difference. Simultaneous submissions are when you query multiple agents about the same project. This is fairly common (and recommended, to a degree).
Multiple submissions is when you submit multiple projects to the same agent for consideration. This is almost never recommended as pitching multiple projects usually comes off as amateurish.
Happy Tax Day, by the way.