Should I not file the contract with a household member then?
You can. But as mentioned above it should suffice if you just use one I-864 and combine both your salaries/income as long as you have lived in the same household in the last 6 months and your work was pursuant to legal authorization from USCIS. If you were working without permission you will need to get a co-sponsor.
We got married in September 2012 (less than 6 months ago, obviously) and moved in a few weeks before that into the same apartment. I've been working with permission
Its probably best to be absolutely sure you should schedule a consult with my office to review your situation before your file. reds