[dragonwriter]

oobabooga/text-generation-webui already supports this for local models (but doesn't support other hosted models.)

SillyTavern technically supports this, though its not a central use case, so the UI isn't particular convenient for mid-conversation change of models, but it works.

But I don't know of anything where it is a central feature and convenient (and if it was, you could also implement things like parallel requests and choosing which response to keep.)