Actually double and triple techs in general would be cool.
Tamamo (Caster) + Tamamo (Lancer) + Tamamo Cat
Gil + Ozy + Iskander
Drake + Blackbeard + Anne & Mary (either version)
Liz + Halloween Liz + Carmilla
Stheno + Euryale + Medusa
Lancelot (Berserker) + Lancelot (Saber) + Mashu
Lancelot (Saber) + Gawain + Tristan
Saber Alter + Santa Alter + Saber Lancer Alter
Cursed Arm + Hundred Faces + Serenity
Kintoki (either version) + Shuten + Raikou
That said I'm actually having trouble thinking of NPs that would combine neatly beyond "they attack at the same time".
Except Blackbeard+Drake and Blackbeard+Yuris, those are obvious.
Lily
Technically already her own Alter form, if you want to take what was done for GO and what Urobuchi wanted to do for Lily.
But, I should have specified with "can't have a SSR Saber without her corresponding SR Alter form".
arash, using stella, firing spartacus, who then explodes.
Are there even any times in the original works when 2+ servants do a combined attack? Right now I can only think in Arthur+Arash, but it's not much more than just "beam harder"