How can I combine the two panels in a duplex on one meter?

Well, if you have 2x 200A service, feeding that from a single meter would require a "class 320" meter (320A continuous, which is 400A derated 80%, so it matches, even if it does not appear that way at first.)

The odds are (since those are a significant step up in expense for the meter can) that you don't have that for the meters you have now. So you'd be looking at replacing the meter, with any upgrades required for the currently adopted code in your area, which include adding an outside disconnect if it's NEC2020. This could be rather expensive, will require permits and (probably) professionals and the inconvenience of having no power midway through the procedure.

As such, it will probably take a LONG time to pay off if the thing you are trying to get rid of is the monthly service fee on the second meter. Combining the bills should be easy, but may not reduce that any - could not hurt to ask, though.