I believe if Jamal had wanted to hit both those shots, he would have. There are two major points in the film that show his skill and his motivation to do so.
Skill: Shortly after the Jamal transfers to his new school, Mailor-Callow, he gets into an altercation with another team member during basketball practice. The coach settles the dispute by having Jamal and the other player shoot free throws until one of them misses. After 50 consecutive hits by both players, they call it a draw (personally, I think he could have kept on hitting, but that's not the point). This shows us that Jamal has complete control over free throws, especially when his pride is on the line.
Motivation: At the beginning of the film, we see Jamal as a basketball player who happens to write. He feels a need to hide his academic performance from his peers (e.g. journaling, telling his brother not to mention good test scores), and he finds refugee in working on his writing with Forrester. As his writing improves we see Jamal transition what he takes pride in, moving from basketball to writing. This reaches a tipping point when Jamal is at the free throw line for the state championship. At this point, Jamal knows he is a good writer, but he feels a conflict that Mailor-Callow is only interested in him as a basketball player, and not for his writing. So how does he respond?
He walks up to the line bounces two shots off the rim.
Keep in mind, we already know he can shoot under pressure. We saw that in the earlier altercation. What I believe (and this part is up to the viewer), Jamal wants to remove any ambiguity about his reason for being at Mailor-Callow. This completes his arc of being a basketball player who happens to write, to be a writer that likes to play basketball.
Now, everything in this answer is secondary to a much more important question from the film, which answers your question much better than I ever could...
That's not exactly a soup question, now is it?
