Here is my experince doing this.
My account setup. 2 iPhone 4 and one flip phone on NATIONWIDE TALK & TEXT FS 700 with the 2 iPhones on unlimited
Only the standard account was eilg for upgrade. I upgraded that line and they had to activate the iphone on that line, and the store wouldn't switch it back for me. I went online and switched the standard account back to the flip phone, and turned off the data plan. I then tried to activate the 5 on my line to replace my 4, and it said there was a issue. I then called VZ and they said the sim card in the 5 was now tied to the account it was orginally activated on and I needed a new sim card to activate it on my line. They also said the sim card shouldn't be perm tied to that line, and she wasn't sure why the phone couldn't be switched. I went back to the store, and got the last sim card they had and my 5 is now on my account with unlimited. The VZ emp said something like the sim cards are worth gold today.
So - it works using a 3rd line, but you might have sim card issues. I upgraded at a VZ store and they force you to activate. If you upgrade at a apple store, I am not sure if they make you activate the phone.