Much like co-signing for any loan, you both will be tied to the performance of the loan itself. Payments made on time will help you both and missed payments will mark your credit and reduce credit scores.
The only way this can be changed is for one of the borrowers to refinance the loan in their name alone, some time in the future.
As pointed out below, the mortgage deduction can be split in any way you both choose as long as it does not exceed 100% of the benefit. You may split the benefit 50/50, 70/30, 90/10 - however you may agree to handle it is fine.
As co-owners on the property, don't forget that non-payment of property taxes can impact you as well. Non-payment can lead to tax liens and could eventually the loss of the home if not handled properly.
On the plus side - having a mortgage paid on time can do wonders for your credit. Over time you will see your credit score improve and future creditors will look favorably at the depth of your credit.
Hope this helps - have a good weekend!