I have a friend on north america so can i play BG duos with him if i am in Europe?
Yes you have to log onto NA servers to do that though. You can switch the server location on your Battle.net client abote the big blue play button, click the little globe on the right of it, it should tell you what region you are in under the play button.
On your phone you have to click bring up menu and click “switch accounts” and choose americas and log in on there.
Edit: do note that your card collection will not travel with you, but for BGs you don’t need your collection.