Curious - why not just each have a card?
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The phone is shown as "installment amount", and that amount is included in the statement balance. You would just pay off the whole balance.
It would not make sense to pay off the phone (0%) while leaving a normal balance (xx.yy%) unpaid and accruing interest.
Two bills to pay, two sets of expenses to track. My wife and I have joint everything when it comes to finances, makes things much easier. We don't have any problems paying for anything but we like to know exactly where we stand in terms of our monthly outgoings and makes it easier to watch for unauthorized charges.
Usually I take care of all the financial stuff anyway, so helps if I can see it all.