Artifacts theming

it seems paired artifacts from the same era give +1 theming bonus
but sometimes its +2 and i cant grip on the rule
its not the same civ bonus as i had barb and hun ancient artifacts which combined were giving +2

As far as I know you get +1 theming bonus from having two artifacts from the same era and +1 for having two artifacts from the same civ (for museums, that is).

It is civilization. You get +1 if they are the same era, and another +1 if *either* both are from a foreign civilization, or if both are from *your* civilization.