I've done back-to-back (ish) tours and think it's a great idea, especially in terms of maximizing air fare, provided you build in a little downtime. RS tours are active (mentally as well as physically) and you're going to be tired. Also, you want to be sure to have some wiggle room for travel hiccups.
The Sicily tour (which I did in 2017--wonderful tour!) ends in Catania, but is there for barely 24 hours. I, personally, wouldn't have minded another day or two there. The same goes for Poland (I haven't done the RS tour, but I've visited twice on my own and loved it). You may want a couple of days to explore on your own. The RS tour starts in Gdansk, I believe, and you may decide that you'd like more than two days there. It's a beautiful city with a lot to do and also some interesting places nearby (Gdynia and Sopot come to mind; Malbork is covered in the tour, I think).
Lastly, depending on the time of year you're going, Poland may be a lovely break after Sicily. I was in Sicily in mid-late June and, wonderful as it was, it was hot!
However you do it, enjoy Sicily and Poland. I did!
Caroline