Georges,
First of all sorry for my bad English.
I also got the same problem on my girlfriends iphone but managed to fix this.
This where my steps: (Primairy device: My iphone6s IOS9.1, Secondairy device: Girlfriends iphone6s IOS9.1)
1. Secondair device settings -->Homekit--> change--> Remove all homes
2. primariy device -->settings-->homekit-->home-->change-->remove the secundairy user
3. Remove Hue app on secondair device
4. Install the hue app back again on your secondair device
5. Login on your hue app with a differend account then the main account that allready is connected on your primairy device (admin)
6. go to your primairy device --> settings siri -->select all scenes you wanna share--> right top --> typ the mail adress from your secondair device from step 4 and send request
7. Accept the request on your secondair device
8. check on your secondair device : Settings-->Homekit--> check if the shared home (configured step 5) is now the only home you see
9. test siri on your secondair device saying the following commands:
• Turn on my [lamp name] (e.g. “Turn on my Sofa lamp”)
• Turn off my [lamp name] (e.g. “Turn of my Reading lamp”)
• Dim my [lamp name] (e.g. “Dim my Bedside lamp”)
• Dim my [lamp name] to [xx]% (e.g. “Dim my Bedside lamp to 90%”)
• Set my [lamp name] to [color] (e.g., “Set my Cooking lamp to orange”)
So the main problem is that homekit default made a "home" config for you. When you accept the sharing from your primairy device the "shared home" gets the secondair config in homekit.
Homekit can only see devices from your "primairy home" so you need to delete the default emty "home" config so the shared "home" config gets primairy in home kit.
Hope these settings also gonna work out for you guys.