You can find it here known as One Ball Live. I have also seen it as One Ball-Two Ball. IMO it is just easier to call it One Ball and be done with it.
Four players begin on court at service line. They begin play with two balls, playing singles straight ahead with the opponent directly in front of them. As soon as someone misses, they call out 'One Ball!' and the point is played out as a doubles point. Notice they continue playing - they do not stop between the switch from straight-ahead singles to full court doubles.
So this game calls for consistency and control while playing half court singles straight ahead, and quick thinking when One Ball is called. Points may be incorporated into this activity. Points are only awarded for the doubles point. The singles point error reducing play to one ball is immaterial and does not result in any points or penalties.
To increase the level of difficulty, have players begin at the baseline.