Can I have more than one TN visa simultaneously?
Yes, individuals may obtain TN visa status to work for more than one employer at the same time. As the initially issued TN status is employer specific, a second TN application would have to be filed for this new employer.
Canadian citizens can file for concurrent TN employment authorization at the border. They should inform the inspecting officer that they are seeking concurrent employment, and want to retain their initial TN employment authorization. However, border officers may allow a TN visa worker with one full-time job to obtain a concurrent TN for part-time employment only. See http://goo.gl/dHOVf. Canadian citizens may also file for a concurrent TN by mail using Form I-129. See http://goo.gl/clFf.
Mexican citizens may also file for a concurrent TN by mail using Form I-129. Alternatively, they can file for concurrent TN employment authorization by filing another DS-160 application at the U.S. consulate. A U.S. consulate will not issue two, concurrently valid TN visa stamps in an individual's passport. See 9 FAM 41.113 N3 at http://goo.gl/rF3Nk. Applicants may, therefore, need to request a single TN visa stamp to cover both TN employers. See, e.g. 9 FAM § 41.53 N8.4-7 at http://goo.gl/j7fpE.