Two TNs at the same time?

I am currently on a TN with one agency and I have been offered a new job with another agency. Do I have to leave the country and re-enter on a new TN or can the new TN be by mail. My current TN is good till the end of the year.

You can mail in an I-129 and add the second company and start working for them when its approved. Processing times for TN renewals are now at light speed. The last TN petition I sent in was approved in 1 day.