Recommendations for time spent in Split and Dubrovnik

How much time do you have?

I will have ten days in Croatia and, in addition to Dubrovnik and Split, I plan on visiting Rovinj and Plitvice.

Oh, is that included in your days? You’ll get some more feedback if people understand your itinerary in a broader context. Because “I have ten days in Croatia and want to visit these four places” is a different question.

Is there any specific direction you want/need to travel in (related to flights, for example)?

Travel between the 4 places you want to visit will be non-trivial, especially if you are not
planning to rent a car. Probably a half-day per leg, maybe more to get to Rovinj.

Are you hoping to day-trip Plitvice or spend a night there?

If you know how long you're planning to spend in Plitvice and Rovinj, then another way to
approach this rather than wistfully thinking about how much time you'd like to spend, is
to figure out how time you'll have to spend. Not the 10 days part, but, day-by-day, factoring
in travel times. And, for instance, does your 10 days include the arrival day (or half-day) and
the last day (before you go to the airport)?

Finally, you haven't said anything about your interests, so you're leaving it to others to guess
what you like to do on your holidays.

Keep in mind that 2 nights in a place often translates into a single day at the place.