Question

What did Nanahuatzin and Tecuciztecatl agree to do?

Answer 1

quite ambiguous. i suggest rewriting your question to make it a lil' bit clearer.

Answer 2

If this is referring to Aztec mythos, then Nanahuatzin and Tecuciztecatl were both given the chance to become the next sun. Nanahuatzin was a humble god and Tecuciztecatl was a rich and powerful god, but to be the next sun, both had to throw themselves into a huge fire.

Tec wanted to do it and stepped forward first but hesitated four times because he feared the fire. So humble and sickly Nana stepped up and bravely jumped into the fire instead and became the sun. Shamed by this, Tec jumped right in afterwards. So suddenly there were two suns, but the other gods were upset by Tec's heistation from before, and so threw a rabbit at his face to dim his light turning him into the Moon.

Source:
Masks of the Spirit: Image and Metaphor in Mesoamerica
http://books.google.com/books?id=soO8ibn7ORYC&pg=PA112&lpg=PA112&dq=nanahuatzin+and+tecciztecatl&source=bl&ots=Jml1b6QhxM&sig=FnUmmWwZdfGUfQtTl5RPTlvqLCQ&hl=en&sa=X&ei=Jb0tUJH1JOe06wH47YDQDA&ved=0CFQQ6AEwBjgo#v=onepage&q=nanahuatzin%20and%20tecciztecatl&f=false