You can have multiple employers.
Found at:
http://ecfr.gpoaccess.gov/cgi/t/tex...div8&view=text&node=8:1.0.1.2.18.0.1.6&idno=8
Title 8: Aliens and Nationality
PART 214—NONIMMIGRANT CLASSES
§ 214.6 Citizens of Canada or Mexico seeking temporary entry under NAFTA to engage in business activities at a professional level.
(i) Request for change or addition of United States employers —
(1) Filing at the service center. A citizen of Canada or Mexico admitted into the United States as a TN nonimmigrant who seeks to change or add a United States employer during the period of admission must have the new employer file a Form I–129 with appropriate supporting documentation, including a letter from the new employer describing the services to be performed, the time needed to render such services, and the terms of remuneration for services. Employment with a different or with an additional employer is not authorized prior to Department approval of the request.
(2) Readmission at the border. Nothing in paragraph (i)(1) of those section precludes a citizen of Canada or Mexico from applying for readmission to the United States for the purpose of presenting documentation from a different or additional United States or foreign employer. Such documentation shall meet the requirements prescribed in paragraph (d) of this section. The fee prescribed under 8 CFR 103.7(b)(1) shall be remitted by Canadian citizens upon admission to the United States pursuant to the terms and conditions of the NAFTA. Citizens of Mexico may present documentation from a different or additional United States or foreign employer to a consular officer as evidence in support of a new nonimmigrant TN visa application.
(3) No action shall be required on the part of a citizen of Canada or Mexico in TN status who is transferred to another location by the same United States employer to perform the same services. Such an acceptable transfer would be to a branch or office of the employer. In a case of a transfer to a separately incorporated subsidiary or affiliate, the requirements of paragraphs (i)(1) and (i)(2) of this section will apply.
