I'm thinking that maybe it isn't so bad that everyone knows that Terry has the Idol. Next week, if Terry wins immunity AGAIN, things will be crazy. The Casayans won't be able to vote for Terry, but they'll be afraid that Terry has given Sally the idol. In which case, the boot is going to be whoever Terry & Sally vote for. If SALLY wins immunity, it'll be just as bad, because then they'll know for SURE that Terry has the Idol.
In other words, the Casayans will have to begin cannibalizing their own tribe, which is going to make them nuts and throws alliances out the window. If Terry and/or Sally can't take advantage of that situation, they don't deserve to win the mil.