VCC1 ( R2 / (R1 + R2) is the equation when VCC2 is shorted. VCC2 ( R1 / R1 + R2) is the equation when VCC1 is shorted.

You V1 = VCC1 ( R2 / (R1 + R2) + VCC2 ( R1 / R1 + R2)

If R1 = R2, V1 = (VCC1 + VCC2) / 2