Canadian with 2 US employment offers, needing TN advice

I recently completed an 8-month internship in the U.S. for "Employer A" and will be returning to the states in January for a 4-month internship with "Employer B". After that, Employer A would like to rehire me for the rest of the year. Technically speaking, Employer B is an organization in corporation with Employer A (i.e., Employer A writes Employer B's paycheck). Is it possible for my employment letter to be written in such a way that I could be granted one I-94 TN form for the whole year or do I need to apply for one with Employer B in January and apply for another one with Employer A in May?

Is it possible to get a 1-year TN and then petition to change employers?
If I do need separate TNs should I return to Canada and re-enter or use the form I-129?
Is the real fact of the matter that if I go to the US with TN status for 1 year, all of my paychecks need to be signed by the same employer?

What is the corporate relationship between A and B? Its not clear from your post. Is one a parent company and the other a subsidiary or branch office? Are they affiliate companies?