If you're looking for something that cheap, go with T-Mobile, and live with only having one phone. You can get a plan for $19.99/mo that has NW calling & roaming, and 60 anytime mins & 500 weekend mins. Or $29.99/mo for the same features, but 300 anytime mins & unlimited weekends.
Prepaid is generally crap and more expensive. It could be nice for infrequent use, except that every provider I've seen expires your minutes after a certain amount of time, so if you don't use 'em, you lose 'em. The longest I've seen was a promo T-Mobile had, in which the minutes didn't expire for a year.
On the other hand, if you rarely use your home phone, you could axe it, and just get a family plan. $49.99/mo will get you two phones with NW calling and roaming and 500 shared whenever minutes, plus unlimited nights, unlimited weekends, & unlimited calling to/from other T-Mobile customers. For this, though, I'd definitely make sure you have good signal where you're at.
As for having two phones with the same number, I don't know any provider who will do this. The best you could do is have two phones with two numbers, but use one as the primary, and just have it forward calls to the other phone instead of to voicemail. Then, the other phone could forward to voicemail.