So I'm downloading a file from a magnet link and got another magnet link from which i want to add peers from to the same download. How would I do this?

Magnet links are based on torrent hash, simply add the magnet link like you would any other it will match if its the same or it will make a new torrent if its a different hash.

Note the hast (if both are using the BASE 16 or both are using BASE 32) will be the same, only the trackers on the end (if any) will be different, other wise its a different "torrent"