Joint equidistribution within 4 modular surfaces and 2 spheres

We will discuss an equidistribution problem concerning rational planes in four-space. Due to an accidental isomorphism this problem relates in a natural manner to the simultaneous study of four CM-points on the modular surface and two points on the two-dimensional sphere. Using Duke's theorem and a joining classification we obtain joint equidistribution under suitable congruence conditions on the covolume of the planes.
This is joint work with Menny Aka and Andreas Wieser, and relies on a joint theorem with Elon Lindenstrauss.