This is commonly called straddle milling, works best with a pair of milling cutters with side rake, one cutter is left handed, the other is right handed, they cut more freely than side mills with the teeth having no rake; the way the cutters shown (stagger tooth) cut in the same manner as the pair that I mention, but only half of the teeth are doing much cutting; stagger tooth cutters are best adapted for cutting slots, having side rake on both sides, the finish better and the alternate spiral of each successive tooth tends to suppress chatter, which side mills tend to do.
Horizontal milling is my favorite kind!