26 C.F.R. § 31.6011(a)-8
The Commissioner may authorize the use, at the option of the employer, of a composite return in lieu of any form specified in this part for use by an employer, subject to such conditions, limitations, and special rules governing the preparation, execution, filing, and correction thereof as the Commissioner may deem appropriate. Such composite return shall consist of a form prescribed ty the Commissioner and an attachment or attachments of magnetic tape or other approved media. Notwithstanding any provisions in this part to the contrary, a single form and attachment may comprise the returns of more than one employer. To the extent that the use of a compsoite return has been authorized by the Commissioner, references in this part to a specific form for use by the employer shall be deemed to refer also to a composite return under this section.
26 C.F.R. §31.6011(a)-8