If you are correct in your assessment that you have an Orange and a Chestnut, breeding them is a good first step towards getting an Orange doe.
I believe the phenotype for an Orange is A_, B_, C_,D_,ee.
The phenotype for a Chestnut is A_, B_, C_,D_,E_.
If your Chestnut doe is Ee, then there is a chance you could get some orange kits. However, at the very least, all of the resulting kits will carry the non-extension gene "e". If you breed one of those does back to the Orange buck you could get some orange kits; since you know that particular doe would carry a non-extension "e".
Bonus if it's a Chestnut doe! The more dominant genes in the A-D genes the higher the chance you'll end up with an Orange