Sorry, I should have said "double," not twin, but I do think the selection of McGregor to play both brothers is a way of making the viewer aware of their intimate relation with each other.
What's important to note is that even Jacob and Esau (though they are twins) are said to look very different from each other. Jacob is smooth skinned (Emmet), while Esau is hairy (Ray). Ray has a mustache and long hair and is a man of the world. He is deals with the riffraff of the world (ex-cons) and his car, though thought to be valuable, is very dirty. Emmitt, however, has a cushy job indoors (Jacob is much more domestic than Esau.).
Varga even directly references the Jacob and Esau story in one of the episodes when speaking of the smooth skin of one brother and the other being hairy.
Another parallel is the episode when Ray tries transforms himself into his brother to try and steal the thing of value (the stamp) at the bank but finds that he has only managed to reap the dirt (the ashes of a dog). This recalls Jacob clothing himself in animal fur to fool his father into thinking he is Esau. But while Emmitt was successful in tricking his brother, Ray's attempt to be the trickster falls flat.
Now, the connections are not uniform, one-to-one correspondences with the various OT stories referenced in this season, but I think they are overt and that is why Ewan McGregor plays both brothers. We are supposed to see them as very much as "doubles" or "twins."