Trip to Rome and Florence, is a 2-1 split reasonable?

With the caveat that Rome is my favorite city in Europe and I believe you can never have too much time there...I think your split is good given your interests. I assume you'll be staying in Florence proper and not venturing out to the Tuscan countryside?

The fast train from Florence to Rome takes only 90 minutes to 2 hours, so if you don't leave for Rome until midafternoon you can enjoy most of a bonus day in Florence.

3 nights for Florence are good.
4 nights for Rome are good.
If you have more (in either), you can spend that time for day trips to nearby towns outside those cities.

That is a decent overview. It will get you started and plan on returning. I often think of Florence as a small Rome. Quieter and easier to adjust to so the split is OK. I am in smay665's camp. Over the years we have spent at least a month in Rome and still haven't seen everything and probably never will. Keep in mind in January the days will be shorter and outside activities will be more limited because of no sun. It will be chilly. So you do need to consider more indoor activities.

3 nights in Florence is good if no day trips
Add a night for any day trips you might add

I would consider a stop 1 night -in Orvieto on the way to Rome
Days are too short for that day trip
Small town between two cities to break up the busy days

It’s fine. That’s what I did in spring

I like the 2-1 split. Roma has so much to see that it could take a week to do it all.

I agree with Orvieto. If you are a lover of Renaissance art, I would spend additional time in Florence.