Those with multiple phones - more than 1 line? Switch back and forth on same plan?...

[mjd9ball]

I have the opportunity to use my wife's upgrade to get a droid - don't yell yet, I'm not bailing on my TP2. As a gadget freak I really want to be able to take a shot with the droid. But I don't want to get another line or plan. I know I can activate phones on VZW myself but don't know if there's a limit to how many times I can do that, if I want to go back to the TP2 full time, switch it to my wife's line, etc... Those of you who have more than one phone on VZW, how do you do it? What's the best way?