Cross-synthesis is generally concerned with impressing the spectral envelope of one sound on the flattened spectrum of another.
Let's call the first signal the ``modulating'' signal, and the other the ``carrier'' signal.
A classic example is for the modulator to be voice and the carrier to be a spectrally rich sound such as wind, rain, creaking noises, or musical instrument sound.
Example: A ``talking organ''
Commercial ``vocoders'' used as musical instruments consist of a keyboard synthesizer (the carrier sounds) with a microphone for picking up the voice of the performer (to extract the modulation envelope).