The gods can have one, why can't we?
I think after you've won a thousand fake gods matches or reach 50,000 points or collect six of every rare or something extraordinary, you ought to receive the option to get a double mark. Either you could pick a double life or earth or whatever mark, or you could pick a combo mark, like life/air or time/entropy. You should still be able to switch marks as you please, and keep the double feature. This would enable even harder AI/decks and handicaps to be developed to continue rewarding players ever higher...