"Clive Davis has actually chosen them a song. He's the man with the golden ears. He's chosen a couple of songs for them to sing. I won't spoil the surprise of what he's chosen for them," said Lythgoe.
"[Archuleta and Cook have] actually chosen a song each from the Top 10 songs that have been voted for [online] and then they're doing another song that they've either sung during the series or something that they want to sing. I can tell you now that David Archuleta is going to sing 'Imagine' again, which is fantastic."
Home viewers will vote for either Archuleta or Cook immediately following Tuesday night's episode, with the winner being revealed during Wednesday night's bloated two-hour live seventh-season finale.
"We have got probably the biggest star in the world right now coming on," teased Lythgoe. "That's all I'm saying -- the biggest star in the world right now."
In addition, Lythgoe said producers also put together another performance via the wonder of technology.
"I've done another one of those videos, if you remember we did Celine Dion and Elvis Presley [during Idol Gives Back last spring]," said Lythgoe. "I've done another one of those, which is absolutely fantastic... More than two [people] this time."
Lythgoe also hinted that Idol's seventh-season finale will feature some "young brothers" -- an apparent reference to the Jonas Brothers.