Combining Rome and Florence in one trip is one of the most common requests. It works well if you avoid “doing everything”: better one intense city day, comfortable private transfers and at least one day for a village (San Gimignano, Volterra) or a landscape outside the centre.
Food helps keep the narrative thread: a Roman trattoria, a Tuscan lunch, a winery stop. Even an unusual stop — such as the Sacred Grove of Bomarzo on the way back toward Lazio — can give the itinerary character.
If you want a programme already balanced between cities and hills, take a look at Taste of Rome & Tuscany.