I noticed that too about the lions two tails. I found this that "may" explain it, but it looks like no one knows for sure why the Czech coat of arms lion has two tails.

It is not easy to be sure for lack of historical sources. Vladislav II. got a first sign with a lion from Friedrich Barbarossa in 1158 together with title "king" as a reminder that Czech warriors fought for Friedrich "as lions". But this lion had just one tail. Where we got the double-tailed lion? Maybe this comes from Premysl Otakar II. He was using a double-tailed lion sign to hint that he is "a second or younger king" of Bohemia (1247). When he was crowned as the king, he kept the double-tailed lion as the Czech coat. And so it remains until today...