I happened to get my green card less than a month ago BH.
If I got your question right than basically the way it works is that the person who is applying for the green card has to have a sponsor that sponsor needs to make a certain amount.
The easiest is if the spouse himself is the sponsor but if he doesn't make enough than you need someone to be the co-sponsor which would be the person who will support you in the case that you spouse can't.
The co-sponsor needs to send in all his tax returns and stuff (I had a lawyer so I'm not sure exactly what but I'm sure you can find the info online) so I would advice you to start looking for someone if you need it as it took me almost 2 month just to find someone (uncle) that was willing to send in all these papers and then 2-3 weeks till he got all the paperwork together
And just to clarify, in your case the applicant (the one filling out the i 130) was not making enough? Or do both need to be making 125%?
From the site it seems that both need but logically that would negate the need for the co-sponsor entirely.
Again in my case my wife is not making enough to even file taxes in the first place.
EDIT: Found the answer here
http://www.immihelp.com/affidavit-of-support/joint-sponsor.html
