Am I obligated to return the money to the two men who loaned it to me, knowing that they do not expect it back and that the second man is difficult to reach, and what should I do to expiate my sin without hardship?

As long as the money has entered your possession as a loan, your obligation is not discharged except by repaying the debt or by the lender's absolution. Communicate with the first person to inform him of your desire to repay the debt. If he absolves you, then ask him to direct you to the second person and repeat the same process with him.