Romulus could also probably qualify for this, I think. In one of his Interludes, he chain-summoned every Roman Emperor/King into Nero's time-period by virtue of just "being there". They aren't summoned "servants", but they are chained summoned. https://www.youtube.com/watch?v=pNF63TM6ago
Doesn't Gilgamesh also summon his Archer counterpart during the final battle in Babylonia using himself? (I'm an anime-only, so correct me if I'm wrong). MutantNinja3636 (talk)