A person can only act as guarantor if the creditor, debtor, and type of debt are specified. Therefore, if two people are owed by someone and another person says, ‘I act as guarantor for paying the debt owed to one of you’, then him acting as guarantor in this case is invalid (bāṭil) as he did not specify whose debt he is acting as guarantor for.
Also, if someone is owed by two people and another person says, ‘I act as guarantor for paying you the debt owed by one of them’, then him acting as guarantor here is invalid as well as he too did not specify whose debt he is acting as guarantor for.
Similarly, if someone is owed, for example, a quantity of wheat (say, 10 kilograms) and a quantity of money (say, £10), and another person says, ‘I act as guarantor for one of the two items you are owed’ and does not specify whether he is acting as guarantor for the wheat or money, it is not valid.