Say there are three points:  Point #1, Point #2, and Point #3.

Say Point #2 is being observed by both Point #1 and Point #3.

Given the Schwarzschild spacetime geometry below,

**  c^2 (1 - 2 U) dt^2 - dr^2 / (1 - 2 U) - r^2 dO^2

Where

**  U = G M / c^2 / r
**  dO^2 = cos^2(Latitude) dLongitude^2 + dLatitude^2

How do you write an equation to include both Point #1 and Point #2
relating the spacetime geometry at Point #2?  Would the one below be
correct?

**  c^2 (1 - 2 U_2) dt_1^2 - dr_12^2 / (1 - 2 U_2) - r_12^2 dO_12^2 =
    c^2 (1 - 2 U_2) dt3^2 - dr_32^2 / (1 - 2 U_2) - r^32^2 dO_32^2

Where

**  U_2 = Gravitational potential at Point #2
**  r_12 = displacement from Point #1 to Point #2, etc.