In the context of a sponsorship application a cosigner agrees to assist in the financial requirements for sponsorship.
Ie.
If, in the future, you want to sponsor your Parents, who have 2 dependent children. You must meet a minimum income requirement to provide for them and their dependents as well as your own family. You wife would act as a cosigner, and your two incomes would be totaled for the purposes of determining financial eligibility.
Thus,
You cannot cosign on a spousal sponsorship because you can't cosign to sponsor yourself.