1537. Contribution to the payment of a solidary obligation is made by equal shares among the solidary debtors, unless their interests in the debt, including their shares of the obligation to make reparation for injury caused to another, are unequal, in which case their contributions are proportional to the interest of each in the debt.
However, if the obligation was contracted in the exclusive interest of one of the debtors or if it is due to the fault of one co-debtor alone, he is liable for the whole debt to the other co-debtors, who are then considered, in his regard, as his sureties.