Oberon and Morgan when they're put in the same party #FGO They actually work well together but the atmosphere on the field must be something to behold...
Nothing compared to putting Rasputin/Kirei in a team full of: EMIYA (Assassin), Muramasa, Ishtar, Manannán and Cú Chulainn.
The level of hatred and anger present in them towards Kirei would be palpable.
For something milder, try putting an Achilles in a team sandwiched by two Penthesilea. It's an amusing combo for sure, especially when you pick the angry skins.