It is Chapter Twenty-Two where you can find the answer to this question. When the Abbot sees that Hank has arrived, he is overjoyed at having a "magician" whose magic has already shown itself to be greater than Merlin's. He thus entreats Hank's help to cure the well. However, Hank refuses to help whilst Merlin is working on solving the problem, no matter how hard the Abbot tries to tempt him to help. Note his reasoning:
"It will not answer to mix methods, Father; neither would it be professional courtesy. Two of a trade must not underbid each other. We might as well cut rates and be done with it; it would arrive at that in the end. Merlin has the contract; no other magician can touch it till he throws it up."
Thus we can see the kind of thinking that governs this response from Hank. From this perspective, if Merlin has the job, it is only fair to let him try to do it and then to take it on only if he shows himself unable to achieve success. Having two people trying to solve it with their own approach and methods would just create a disaster. Thus Hank is happy to let Merlin finish his attempt at curing the well, before he will step in and apply his "magic" to the problem.