Hi GWHITF,
The rental of camera bodies, lens and lighting gear is standard practice in commercial and advertising photography. (If the budget allows) I do a mix. Sometimes I use my base gear of MFD and other times I may rent a different back or some exotic glass.
One thing to be aware of is that the Photography rental house requires the photographer or producer to have a insurance rider to cover the amount of gear rented. If you have been commissioned to do an advertising gig, there is a line item on the bid or estimate for "Insurance". So that means the production will cover the extra costs of insurance.
On the other hand if you own or have in your personal kit two MFD backs, then your "own" business insurance would have to cover the your gear. And let me tell you that can be expensive....
Regards,
Jeffery