You do not need two phone plans, assuming you only want one phone active at a time.
You can put a razr, a flip phone, an I-3, I4 or I5 on that line anytime you want. With the changes in SIMS etc, it may not be as easy as switching them out.
Personally, I would think you are looking for some real heahaches with recent calls, texts, emails, etc by doing this on a regular basis.