Combining two or more curves can result in movement that isn't describable using Bezier curves. We'd need to bake the resulting animation, meaning to set a key every X frames. This would result in many keys. That isn't ideal for performance and makes it difficult to edit. The feature could still make sense if we also had tools for simplification, which replaces multiple keys with fewer keys that describe (roughly) the same curve.
We've discussed adding these features, but we have other features we want to get to in the short term.
