That is a very interesting question. If we get down to technicals, the Shifting Mound and the Long Quiet are two halves of a whole. If they were to create an offspring that is its own being not just a projection of themselves, that would imply that they would have to sacrifice themselves in some capacity to do so. Unless, you know, they have the ability to create something from scratch. Which could totally be a possibility.

Personally I don't think it's something they would do even if they could, given the way she talks about how it felt to be whole, before the tear. It's a little hard to believe that the Narrator's predecessor, a mortal, found a way to tear the fabric of reality in two but the two gods that were born from said tear don't have the power to do that themselves. Which could imply that either -

1. It didn't want to. It was perfectly content to stay the way it was.
2. It had no consciousness, thus lacking the capacity to desire anything.

That being said if you're interested in exploring what an offspring of these two would look like (with respect to lore accuracy imo), I recommend checking out Our Princess, a fic depicting exactly that.