2 Curse Blasphemy and 'Whispers of Doom' don't interact

Having a hard time understanding why this interaction doesn't work. It clearly states can add an additional Curse, yet Blasphemy only puts one curse on. I don't see why this can't be possible combo. Any reason for this? Curse already is at like 40% power, don't see why I can't have 2 if I have the keystone passive.