What bonds the two men even closer as friends?

Which two men are you referring to?

Dantes and the other prisoner.

In Chapter Ten, we witness the meeting and the friendship of Dantes with another political prisoner, Abbe Faria. This prisoner has spent years tunneling his way to freedom, and a miscalculation has led him to Dantes' cell. The Abbe is crushed and his energy to escape is as well. He shows Dantes the tools he made and his writing, A Treatise on the Possibility of Monarchy in Italy. He is a very learned man, and thus shares his knowledge of languages, mathematics, and science with Dantes over the next few years. The Abbe also aids Dantes in deducing who was responsible for Dantes' incarceration, and he soon concocts another plan to escape. Thus the two begin tunneling to freedom once again. Before they are able to finish, however, the Abbe suffers from an apoplectic attack, which paralyzes his arm. He informs Dantes that he now expects death very soon, and encourages Dante to escape without him (the tunnel is finished). Dantes refuses to leave him.