Error "Please contact itunes support to complete this transaction" when in-app purchases

Dear Sirs,

As frequent customer buying iTunes Gift Card, I would appreciate your kind clarification on below points, for my best reference:

- I'm using Apple ID Canada and playing a game (i.e. Clash of Clans from http://www.supercell.net) which identifies each game account by their GameCenter account. Hence in case of using only this Apple ID, for how many account game I can buy in-app purchases?

http://itunes.apple.com/ca/app/clash-of-clans/id529479190?mt=8
- I use my Apple ID buy gem ( intem of this game ) for 2 anccout game ( 2 iDevices) , but now I can't buy more Error "Please contact itunes support to complete this transaction" .How I can fix it ?

For ex: My Apple ID is abc@gmail.com and share to a friend. Both of us log to this Apple ID "abc@gmail.com" to download a game on 02 different iDevices namely "acc1" and "acc2". Is it possible to use only this Apple ID to buy in-app purchases for both account games "acc1" and "acc2".

Looking forward to hearing from you soon.

Thanks and Regards,