How could it possibly be done simultaneously?

Oh, quite simple, The Median was charging with his thirty-foot lance in front of him; the Thracian knocked it aside with his buckler; the point glanced by; then he knelt, received the charge on his pike, pierced the horse’s chest—the spirited beast impaling itself by its own impetus—, and finally ran Arsaces through groin and buttock. You see what happened; it was the horse’s doing rather than the man’s. However, Arsaces did not at all appreciate equality, and wanted to come down on horseback.