Looking at some CPO vehicles and on the MB website, it says CPO extends the factory warranty to unlimited miles and 1-2 years. Does the dealer decide wether you get a one year or two year extension or is that fixed per the model?

Trying to figure out if that might be a negotiating point on the purchase...

CPO is a one year warranty extension. You can purchase one or two additional years at extra cost. The price varies with the model.

You get what remains on the original 5 yr/50K mile warranty, + 1 additional year.