Iphone upgrade program, can I keep and pay off the old phone?

I recently received my new iPhone X under the upgrade program. I was going to return my iPhone 7S+ and be done, but I am considering keeping the 7+, continuing to pay the installments, and give it to my wife to replace her 5S. Can this be done? I mean I have essentially opened a second 24mo loan for the X, and only have a few months before the 7+ would be paid in full. I would just pay off the original loan as well as the new one. I don't believe there is an obligation to return the 7+ as long as I pay it in full. Thoughts?

Karl