Reuven was in need of a loan. The local Gemach (free-loan fund) requires two guarantors, referred to in the loan document as arev kablan. Levi and I agreed to guarantee the loan for him.
When the loan came due, Reuven was unable to repay it. The administrator of the Gemach called me, and I told him that I am willing to pay half the loan. I see no reason that I should be liable to pay the entire amount, since there is a second guarantor, Levi. The administrator insists that I repay the entire amount.
Q: Am I obligated to repay the entire loan?
A: There are two different categories of guarantors in halacha: arev and arev kablan. A standard arev is not obligated to repay the loan until the lender has exhausted all possible efforts to collect the loan from the borrower. If the lender did all that he could and the borrower still did not repay, the lender may then demand payment from the guarantor. An arev kablan accepts a higher degree of responsibility for a loan. The lender may approach him to collect payment even without approaching the borrower first. For all intents and purposes, the arev kablan is considered the borrower.
In your case, since you signed as an arev kablan, the Gemach has the right to ask you to repay the loan even without first approaching Reuven. But there is another matter that must be addressed, an issue that applies even in a case of a regular arev where the lender failed to collect from the borrower: how do multiple areivim share the burden of repaying a loan? May the lender collect the entire amount from one guarantor, or do all of the guarantors share the burden of repaying the loan equally?
This topic is the subject of debate amongst halachic authorities (C.M. 132:3). In the opinion of some authorities, each guarantor is wholly responsible for the loan; the lender can choose to demand payment from any one of them. The essential definition of an arev is that he accepts responsibility for the loan, and if two people serve as areivem, they each become responsible for the full amount of money (also see Sema 77:3). Others disagree and maintain that the lender may not collect from one guarantor when both are capable of repaying the loan. Since both guarantors knew that they were jointly guaranteeing the loan, it is assumed that each one accepted responsibility for only half the amount as long as the each is capable of repaying his half (see Shach 132:4, who seems to agree with this opinion. See also Ketzos 77:3).
Since the matter is subject to debate, the Gemach administrator cannot demand payment of the full amount from either guarantor, whether they are regular areivim or areveim kablanim. In the event that a Gemach wants to reserve the right to collect the full amount of the loan from any guarantor, it should be clearly written in the arev agreement that the Gemach has the right to collect jointly or severally from either guarantor (C.M. 132:3).