While this may work well for you as the co-signer is liable for any default as you are, the coop board may or may not be open to this arrangement. If your co-signer will be living with you in the coop, that may be more acceptable to a boad than a co-signer that does not live in the coop.
In any event, most boards are very particular in regards to people's income and credit. every deal has it's own problems and boards will take these things on a case by case basis. Good Luck.