Dear Pete,
Yes. Amazons and Greys can be compatible... but my answer to you is DON'T DO It. Dukey has been an only bird with you for 6 years and she knows the two of you as being the flock. The new bird will be seen as an outsider. You have also seen how jealous Dukey gets when you switch your attention to human guests. You will have the same trouble with this. She may try to attack Coco or she may go into herself.
I had my Merlin for three years when Sweetpea came to live with us. Merle was SO jealous that she started pulling her tail feathers and yelling "Yick. You're lucky" to Sweetpea when I held Pea. I had to place Pea in another home for a year before reintroducing her. You can get the long story by going to the article section of this site and looking at the "Living with Merlin: Introducing the Second Parrot" articles.
If you decide to do it anyway, PLEASE always treat Dukey as the TOP BIRD. That is, she gets attention first... gets food first... gets out first... gets to bed LAST. I'm sure it may be possible to work it out at home, but it would probably be a long, arduous process... that's probably why you are having second thoughts. Instead, I suggest that you consider making the Coco the Amazon your store mascot and being "your bird" in the day, away from Dukey.
Pete, I speak about this one from experience. PLEASE read my articles on this subject in the article section of this site. If you have any specific issues, please write to me at merlin@AfricanGreys.com
Blessings,
Maggie-