The Gemarah in TANNIS (9a) says that there is nothing in the Nevim and Kesuvim that is not hinted to in the Torah. My question is, why have both?
I think you mistranslated. The gemara says:
מי איכא מידי דכתיבי בכתובי דלא רמיזי באורייתא
Is there anything in the scriptures (Nach) that is not hinted to in the Torah?
This is a common idea. The idea that the Torah, on some level, contains all knowledge. The fact that Shaul would be the first king of Israel, the fact that Sancheirev's army will be destroyed, the Jews will sin until their Beis Hamikdah is destroyed - it's all hinted at somewhere in the Torah. All the lessons of Shlomo in Mishlei, all the history in Divrei Hayamim - it's all in the Torah somewhere at some level.
So why do we need both? Clearly, the average person is not able to find all the lessons and concepts and history that is hinted to in the Torah. We need it "spelled out" for us in the books of the Neviim and Kesuvim.
Precisely because it's not "spelled out" in the Torah. The expression in the Gemara there is רמיזי, "alluded to."
In Nedarim 22b, we are told that "had the Jewish people not sinned, they would have been given only the five books of the Torah and the book of Yehoshua - [the latter] because it is the land survey of Eretz Yisrael." So yes, if we had remained on the high level expected of us, we'd all be able to deduce the teachings of Nach from the רמזים in the Torah. Since we didn't, they had to be spelled out more clearly, and it takes someone of the caliber of Reish Lakish's son (as described in Taanis ibid.) to show their source in the Torah.
I would beg to differ with both previous answers. The Gemara says דכתיבי בכתובי which I understand to mean just the ktuvim. As Tanach has 3 parts, the third being the ktuvim, I understand it to mean just Mishlei, Tehillim, the Megillot, etc. Not the Neviim. The Neviim prophesized to their generations based on the needs of the time. When the people sinned, when things needed changing, the Navi was there to rebuke them and direct them. Like Alex quoted from Nedarim, there would be no need for the Neviim had the people not sinned, b/c there wouldn't be what to rebuke. I don't think these are hinted to in the Torah. However, ethical teachings, ways of life, that is hinted to in the Torah or at least the principles can be inferred from the Torah's laws and guidelines.