I'm so sorry to hear that Whisper is so aggressive towards Shadow. Hopefully the feliway will help you.
I have a similar problem with Storm and Cirrus and have the feliway spray but I rarely use it. When I squirted Storm with water, this seemed to make it worse. Now I just yell at Storm and put him in my room for a time out. Sometimes this helps and sometimes it doesn't. I've learned to accept the fact that they'll never like each other.