Default Question to keep my unlimited plan.

So I'm going to be adding my fiance onto my plan(currently a single line with unlimited data) and am able to keep my unlimited while just getting her a 2gb plan. My line has an upgrade available and I was hoping to take advantage of the BOGO deal for the M8 by purchasing a new one for her line, then swapping the new phone to my line and my upgrade to her line and getting the 2nd one that way. I was told that's a no go.

Is there anyway I can make use of that deal without adding a 3rd unneeded line? Or can I just buy her phone today with the new contract, switch my upgrade to her line in a few days and then go buy another one at the 2year contract price without me losing my unlimited?

Possibly doing this tonight or tomorrow, just wanted to see the best option.