QUOTE(cambiata @ Nov 21 2008, 04:50 PM)

Try Pauline Hall's Piano Time Carols - I think they are the easiest Christmas duet arrangements around and they sound good too

They can't possibly be easier than the John Thompson ones! That's quite a good book (JT's Easiest Piano Duets - see, I told you they were the easiest ones

) and has about five carols, plus lots of other simple pieces that could be played by two pupils, one slightly more advanced than the other.
If you want a book where the primo part is very simple and the secondo fleshes out the music, Fanny Waterman's Two at the Piano is pretty useful, if a little old fashioned now. Also Duets with a Difference and Mixed Doubles (from the Piano Time series) are good. One book is slightly harder than the other, but I forget which way round.