Well, fade the volumes, use some different filters. Maybe high pass filter both sounds so they sound very tiny when they morph, and then lower the filter so you can really hear the new sound.
You could also try cutting the sounds up in pieces of, say, 64ths or 128ths. Then mix them so that in the middle it's one of the first, one of the second sound. And then the first sound decreases so at the end you will only hear the second one.
I doubt anyone will ever decipher my second tip though