Well, not really spoilers, but you can't be too careful theses days. 
This is what they will be singing:
Ché Chesterman
In The Air Tonight - Phil Collins
Valerie - Mark Ronson Ft. Amy Winehouse
Louisa Johnson
Fighter - Christina Aguilera
I Believe I Can Fly - R Kelly
Reggie 'N' Bollie
Spice Up Your Life / Boom Boom Boom - Spice Girls / Outhere Brothers
Jump - Kriss Kross
No Christmas songs
Tomorrow the remaining 2 will sing their favourite song from the series and the winner's single.
The duets haven't all been confirmed- apart from LouisaWHOIS17 singing with Rita, maybe. (Bit of cost cutting there).
Other guests over the 2 nights are rumoured to be-
Little Mix, Leona Lewis, Fleur East, One Direction, Ben Haenow, Coldplay and ADELE . Can't tell whether these are just guests, or singing with the acts.
Can't picture Adele and SpagBollie somehow. Not sure who will draw the short straw there.
Obviously getting Adele and Coldplay has blown the budget, hence the rest being X Factor alumni
I need mulled wine and nibbles to get me in the mood.
see y'all later.
[santa]