I had my park ticket added to my KTTW card this year and everything was fine - AND the CM who did it also gave me back my separate park ticket, so if the KTTW hadn't worked for admission, I still had the original ticket on me (which I assume would also have worked).
Although I never tried it, so maybe not? Anyone know if the original ticket is "deactivated" when they add it to your KTTW?