It wouldn't be incest anymore than if it happened to non-twins.
And the children resulting from both couples wouldn't have any more chance of looking the same as "normal" cousins would, because that is all they would be. Even if there is an extra blood relation compared to "normal" cousins (ie: mums would be sisters AND dads would be brothers, whereas you only get one of each usually), the chances of them looking anymore alike than just cousins are infinitesimal, I think.
Now I'm going to lie down, because I've confuzzled myself....lol