Ron..surely the reason that you are asking this question is that the Mobile phone that you use, is out of order due to its need to be recharged ?
So, the easiest way to combat this is charge the phone, when you are out and about, not to dream up yet wackier ways of using 2 phones, or diverting calls from one to the other, or changing SIMs.
Just go and get an in-car charger, and your problem is solved !
By the way, if you are a light-user of your Mobile, most phones these days last ages between charges