Separate tickets won't magically merge into a single ticket and give you a refund of the difference. If you have a package vacation, you'll need to phone and add a day. If you booked through a Travel Agent, they can make the change for you, if you booked yourself, you'll need to call Disney. I don't think you can make changes online. As said above, you need to make the changes before you go if you want to book FP+ in advance for the extra day.
A 1-day ticket is the most expensive way to buy tickets from Disney. They do this to discourage short visits from splitting between Disney and Universal by making it more economical to just stay at Disney for the duration.
If the FP+ isn't a big deal, you can add the extra day while you're there, just be sure to do so before the two-day tickets are expired [once you use the second day's admission, you can't upgrade the ticket]. so, if you plan on adding, do so when you arrive or while you are in a park already. Your 2-day ticket is valid for 14 days from first use OR when all available admissions have been used, whichever comes first.