It is definitely a mating behaviour. Males do makes sounds whenever they smell any hedgehogs around. They may not know the gender of the other at first until they confirm it. Some males will take time to realize that the one they are trying to mate is a male and when they do, there is great risk of them fighting. Older males who's mojo is more active will usually be the first to make the sound. It would be safer for both to be separated and so that they would not be both stressed. You were lucky that Mushu didn't hurt Stitch when they were together.
Mushu will be fine on his own, really. No matter how lonely they may look to us, they would prefer being alone than with a hedgie friend.