Did you already provide a loan approval letter along with the offer on the short sale? If so, then you would want to submit a new approval letter. Sometimes changing things mid-stream muddies the process, so sometimes it's best to just let it go as is. Ultimately, so long as you can get your loan with the co-signer, as long as you can close your loan and close escrow, the short sale lender shouldn't be affected.
If you have any other questions about short sales, I'm happy to answer them for you. 760-908-3838