I have a punishment hearing that is now on appeal. The Defendant went open plea to the court in exchange for the State abandoning the deadly weapon finding.

At the conclusion of the punishment hearing, the judge announced the sentence as 20 years TDC but "if the Defendant will advise the State where he hid the money and the full amount of money is returned to the victim, then I will reduce the sentence to 18 years."

The appellate lawyer is arguing the alternative sentence is not lawful, the pronouncement of the sentence is not severable, and is void. (Leading to a new punishment hearing)

Any ideas about how I should attack this? Any help would be much appreciated!

what is his authority that they are not severable?

Alternative sentences are not permitted. A recent case from the CCA explained.

I think John is referring to Morris v. State, 301 S.W.3d 281 (Tex. Crim. App. 2009). The CCA found the alternative sentence unlawful and struck the unlawful portion, preserving the lawful portion. So in your case, I'd say the "or 18 years" part is unlawful and should just be struck, leaving you with "I sentence you to 20 years."