Double Summon

Double Summon. The skill possessed by Assassin of “Red,” Semiramis. It is an underhanded skill that is only invoked when certain conditions are met during the summoning ritual, like a “command input on the title screen.”
Of course, this kind of skill has some restrictions, with the three Knight classes (Saber, Archer and Lancer) and some of the extra classes (like Ruler, Shielder and Avenger) incapable of being applied to the Double Summon skill. In other words, only a combination of the Berserker, Caster, Assassin or Rider classes is possible. However, though rare, a modern day extra class like Gunner can apparently be combined with another class through this skill.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia