Yeah, I know you mean. I once asked an EB employee if they had a used copy of FF X-2. The way I said it was this, Final Fantasy ten-two. The guy did not know what the fuck I was talking about. He was asking me what system it was for and if I was sure it was already released. When he finally understood what game I was talking about, he refered to it as ecks-2.
It depends on the context. I call FFX, Final Fantasy 10, but I call FFX-2, Final Fantasy ecks 2. Ten-2 sounds too retarded to say (hmm, maybe that actually suits the game better).
Likewise, I call Mac OS OS ecks because it sounds better than OS 10.
You could always refer to X-2 as Final Fantasy Ten-Volume 2, Final Fantasy Ten-Chapter 2, Final Fantasy Ten-The Final Chapter, Final Fantasy Ten: Electric Boogaloo, or "that really shitty sequel to Final Fantasy Ten".