When you co-sign a loan you are guaranteeing payment.  It does not matter if you are the primary or secondary borrower.  Once you execute the finance agreement you become subject to Joint and Several Liability, which means that upon Default the lender can choose to pursue the primary borrower, the secondary borrower, or both. 

In addition, the lender is not required to collect an equal value from each individual.  So after a lawsuit is filed and a Judgment rendered, they could ultimately secure 100% of the outstanding balance from either borrower via potential wage garnishment and/or bank attachment.

Often co-signers are required for student loans and auto loans, and in such circumstances, only the primary borrower derived any related benefit.  Consequently, that may provide the co-signer the ability to bring a Cross-Claim against the primary borrower during the pendency of the lawsuit on the basis of "Unjust Enrichment".  This may result in the co-signer securing a Judgment against the primary borrower for the sum due.  However, that would not otherwise serve as a defense to the lender's ability to obtain a Judgment against, and collect from, from the co-signer.

Consequently, if you are a named Defendant in such proceedings, and the primary borrower is unable or unwilling to effectively resolve the matter, then it may be advisable to either try and work-out an agreed payment plan or lump-sum settlement with the creditor, or alternatively consider filing for Bankruptcy protection.