It's hard, but possible. Once you get some experience and flight hours with local companies in Poland, you can became a self employed and work as a contractor (RYR eventually works like that) - you basically will "work" and pay taxes in Poland and commute to the base in the other EU country. But again, not all companies allow it. Anyway, I think once you finish your 0-fATPL course and start getting experience locally you already will have a citizenship in Poland, so all EU market will be opened for you.