Bava Batra 173 - Obligations of a Loan Guarantor
If a man says to his son on his deathbed, "One document among my several loan documents has been paid, but I don’t know which one it is" - all the loan documents are treated as paid, since each borrower can claim that it is his debt that is paid. If two loan documents were from a single borrower, the larger one is considered paid.
One who lends money to his fellow on the basis of a guarantor may not collect first from the guarantor. However, if the lender said to the guarantor at the time of the loan, "I am making this loan on the condition that I should be able to collect from whomever I wish" - then he may indeed collect from the guarantor.
Art: Leon Daniel Saubes - Portrait of Victor Hugo on his deathbed