Most of the colleges I’m applying to just want me to report my best ACT score.
However, one of them, Cornell, wants me to report every single one I’ve taken, according to their website (I’ve taken 2).
Does that mean I have to report both on my common app?
Or do I just report the best one on the common app and send 2 scores to Cornell?
Yes, you can choose to only self-report that best score.
As long as you have officially reported (paid to have scores sent) what each school requires, what appears on your self-report page is up to you.