If you’re only signing for your children, you can probably cosign for two loans without much problem, but remember your debt-to-income ratio will also be a factor. Essentially, your debt (including the car loans) cannot exceed 43% of your income.
Keep in mind: If either of your kids doesn’t pay, you are on the hook for the outstanding loan payment(s). Failure to pay will negatively impact both you and your child(ren)’s credit scores. So, have a frank discussion with your kids about the associated responsibility before you sign.