Can someone co sign with you on refinancing a mortgage?

Yes. Odds are good they will have to provide all the same documentation you are going to have to provide and they are of course going to be on the hook for the exact same amount as you. Their credit will be impacted, good or bad, by the way the mortgage is handled.

Same documentation and approval as the primary. Includes can they pay for mortgage iif the primary does not. That means if they currently have a mortgage, they have ENOUGH monthly income to pay the other mortgage that they co-signed for and their own.

There are some lenders that will allow that. It’s not very common.

Yes, it is usually required by some lenders if the principal borrower does not have enough collateral to support the mortgage or as an assurance that the mortgage will be paid. Both the principal borrower and the co-signatory are liable.