I have 2 phones under 1 shared plan. The way I managed it was to make sure I was eligible for an upgrade every year in the month of October, hence, having each phone up for an upgrade alternating years. I'd hand down the 4S to my parents and get the 5. Next year, upgrade the 4S to the 6 and hand down the 5 and so on and so forth.
I think I might get the new iPhone off contract/unlocked at Apple and go by way of prepay. I don't need data so I wonder if I can get that to work with T-Mobile or Virgin Mobile? I'll have to look into it when it gets released.