On their website, it specifically mentions cosponsors for I-864 for the immigrant visa (IR1/CR1), and leaves the mention of cosponsors out of the requirements for K visas with the I-134
http://usembassy.state.gov/posts/rp1/wwwh3217.htmlEVIDENCE OF FINANCIAL SUPPORT
Form I-864 Affidavit of Support (AOS). The I-864 AOS is required for visa applicants in all family-sponsored categories (IR and F) and certain E visa categories. The I-864 AOS is NOW VALID INDEFINITELY ONCE IT HAS BEEN SIGNED AND NOTARIZED (this is a change from the previous requirement of being good for one year only). In other words, applicants will no longer need to submit a new I-864 AOS or tax forms simply due to long processing delays between the date of the signature and notarization of the I-864 AOS and the date of the interview.
Additionally, petitioners (or cosponsors, if needed) filing an I-864 AOS are required to submit ONLY ONE YEAR'S TAX RETURN, CURRENT AS OF THE DATE THE SUBMITTED I-864 WAS NOTARIZED (this is a change from the previous requirement of submitting tax returns from the three most recent tax years).
The actual signed and notarized I-864 AOS must be an original copy. However, faxed copies of tax returns are acceptable.
Click here to read more about the required I-864 Affidavit of Support (NOTE: THE RECENT CHANGES IN I-864 AOS REQUIREMENTS ARE IN THE PROCESS OF BEING UPDATED ON THE MAIN U.S. DEPARTMENT OF STATE WEBSITE)
Form I-134 Affidavit of Support.
The I-134 Affidavit of Support is required for returning residents (SB-1), fiancé(e)s or spouses of U.S. citizens (K1, K3), children of K1 and K3 applicants, spouses and children of LPRs, applicants with special immigrant status, and follow-to-join derivative family members of employment-based (E) applicants. A copy of the sponsor's most recent Federal income tax return (Form 1040) should accompany the I-134. Fax copies of the ITR are acceptable.