I don't even have a phone number for Aimee. Amy is even marked as a favorite. Whenever I say "Call Amy" then Siri says "I don't have a phone number for Aimee."
It was working with iOS 9.
iOS 10.1.1, iPhone 5,1.
If both people are in your Contacts, you can give one, or both, a nickname. When using Siri, you can refer to the person by nickname and Siri will know who you're talking about.
To add a nickname to a person:
add field -> Nickname
To remove a nickname, just reverse the steps. You can change the nickname at any time.
Not really a soultion, but would probably work anyway:
Roll with it - add Amy's phone number to Aimee's contact info!