2D Chebyshev and legndre polynomial.

I would really appreciate your help on this. I am trying to fit a surface with Chebyshev and Legendre polynomials. I can write the polynomials. My question is how to write them in 2D. In other words, the basis I can find only depend on x and not y. How do I have y in my orthogonal basis: These are the basis:

http://en.wikipedia.org/wiki/Legendre_polynomials

Zara