According to myth in The Way to Rainy Mountain, where did the twins come from?

When you speak about "myth," you are speaking about the first of the three voices that Momaday uses in his book The Way to Rainy Mountain.  Just as a short explanation, let me mention that the major parts of The Way to Rainy Mountainare called "The Setting Out," "The Going On," and "The Closing In."  These parts are separated into twenty-four sections with each of them divided into three voices:  one having to...

When you speak about "myth," you are speaking about the first of the three voices that Momaday uses in his book The Way to Rainy Mountain.  Just as a short explanation, let me mention that the major parts of The Way to Rainy Mountain are called "The Setting Out," "The Going On," and "The Closing In."  These parts are separated into twenty-four sections with each of them divided into three voices:  one having to do with myth, one having to do with history, and one having to do with a personal story.  Your question has to do with the mythical voice (always the first of the three voices) and, specifically, has to do with "The Setting Out."

In addition, "the twins" you speak of are very important to the Kiowa creation myth.  While the creation myth has to do with both the bundles of the Talyi-da-i (which means "Boy Medicine") and the Tai-me bundle (the creature-God of the Sun Dance), "the twins" are about the Talyi-da-i, specifically. 

The Talyi-da-i, or "Boy Medicine," part of the myth is about the offspring of the Sun and a Kiowa woman.  The child produced through this union is a son who is given to Spider Grandmother in order to raise him to adulthood on the earth.  Although Spider Grandmother worries about her charge, a miracle soon happens:  the son splits in two, forming twins. Now both twins are considered "sons of the Sun."  It is not long before one of the twins mysteriously disappears into the lake waters in order to become one with the earth.  Nature is now sacred to the Kiowa as a result.  The other twin miraculously converts himself into the ten bundles of Talyi-da-i in order to give a god-like gift from the Sun to the Kiowa tribe.

In conclusion, if you are searching for a very short answer to your question, then I can provide that for you as well.  According to the myth, the twins came from the union of the Sun with a Kiowa maiden.