Fusion ultimates.. is it something Riot has ever considered?

Fusion as in champion X's ult interacts with champion Y's ult to produce an ult that differs from just X+Y?

I doubt it could work out, as Riot has expressed they don't want champs that make other champs almost must pick alongside them due to the synergy they have. Such a fusion ult would either be strong enough to make the other champ(s) necessary, or weak so that other champ(s) would actually be a hindrance.

Plus, champs that require the cooperation of other players don't always feel good, and can lead to some frustrating moments. Take for example kalista ult, where theres all kinds of situations where kalista ults intending for the ally to do one thing, but instead they do another.

By fusion ultimates you mean like if I were to fuse Renekton's and Nasus's ultimates you get two cho'gath sized champs? lol