Em+D
a whole step apartWhat it adds. Em holds the chord — 3 5 7 — and D sits on top of it as 9 #11 13. That second triad is the whole point: three notes, none of them in the chord, and between them the 9th, the #11th and the 13th arrive at once. It is the sound people mean by "lydian" without anybody having to play a scale.
How it sounds. Open and bright. The upper triad puts 9 #11 13 above the chord, and those are the tensions that lift rather than pull — nothing here wants to resolve anywhere. It is the sound of a chord being enjoyed rather than left.