Karta Miejska in Warsaw

I have a Karta Miejska strefa 1 - so valid within the city boundaries.
The thing is that every weekend I am commuting to Piaseczno and back, which is in strefa 2.
I was thinking on the next renewal to change my Karta Miejska to strefa 1 + 2, but that is a big price difference (and after 1 Jan that difference will be almost 100 PLN)

Until now I am solving this problem with buying in advance a train ticket from Warszawa Jeziorki (end of strefa 1) to Piaseczno. And from Śródmieście to Jeziorki my Karta Miejska.

This means on the same train I am using my Karta Miejska and a train ticket.
But I heard from several people lately that this might be illegal.
Is this the case?

It's perfectly legal in Poland.