It's unusual for two people to own one trademark. Because trademark's only exist if they're used in the marketplace to sell something, and because a partnership is formed automatically when two or more people work together to sell something, the trademark owner is usually the partnership.
Be that as it may, follow the directions here < http://tr.im/r4yb > to remove one of the two purported trademark owner's names from the registration application. Better yet, speak with a trademark or business attorney in your state because it's very likely there are other partnership and other legal issues percolating below the surface of which you are unaware.
Presumably this is pursuant to a dissolution of your partnership with this co-owner. As noted by the other response, there is a procedure available to amend your TM application, but you should consult with a qualified IP lawyer to ensure that all your partnership assets are properly disposed of and that you deal wtih the USPTO correctly.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
You need to check with a Trademark Attorney, such as myself, to verify the details, Your story indicates missing details. There is only one owner of a trademark.
