Wouldn't say rare, since they are common corals, but difficult to locate the two on the same piece of rock. Multiple color variations of tubastrea (orange and yellow) come in all the time either naturally grown that way or glued by the vendor (more likely way you will find the black suns mixed with orange this way). Might want to give either Phishy Business or Aquatic Technologies a call since combinations you are looking for have come through their doors every so often.
Mike