Technically, both co-borrowers owe 100% of the loan. For example, if you enter a $30,000 joint loan with a co-borrower, both borrowers owe $30,000. The lender will only seek out what it has lent, though, so there is no need to pay that amount for each borrower. The lender will likely only chase up one of the borrowers for loan repayments at any given time rather than both, but whether it chooses to alternate between co-borrowers will vary between lenders. Make sure you have a clear repayment arrangement with your co-borrower.