Is the second Mudarib obligated to return the capital to the owners of the money in a Mudarabah partnership, considering that he was executing the orders of the first Mudarib, and he has returned the salary allocated to him?

The questioner did not fully elaborate on the terms of the contract. If the agreement was limited to a salary, then it is an ijarah (lease/hire contract), and the questioner has no relation to the return of the capital, nor is he obligated to refund the salary.

If the contract was for a salary and a percentage of the profits, then the contract is invalid according to the majority of scholars, and he is entitled to a ujrat al-mithl (fair market wage).

The hand of the mudarib (managing partner in a profit-sharing venture) is a hand of trust (yad amanah), so he does not guarantee losses unless there is transgression or negligence. Losses otherwise are borne by the capital owners, and the mudarib loses his effort. If the loss is due to the mudarib's negligence, then he is liable for it.