What exactly are you asking?
You say that you have an 'iPhone' that is 'welded' into its case, and that you're thinking of buying another iPhone to use in the car. Right? So does that mean that you never use the 'welded' iPhone in the car? Or does it mean that it is 'welded' to some sort of vehicle mount and now permanently lives in the car? Or something else?!?
If you're asking whether you can use two iPhones with the same "internet provider" (cellular provider), provided that they're compatible with the carrier's network, and that you're willing to pay for two lines, I'm sure they'll be happy to sign you up for two lines.
Or was your real question whether you could get cellular Internet service on a second iPhone without having to pay any extra money to your carrier?
If you get a second iPhone, is there any reason why you would keep using the 'welded' one? If you're going to get a second one, why not just use it for everything?
As you can see, I don't have the answers ... just more questions, for you to consider.