I've worked that out in my head. In mine, it's just a simple file folder. Opened one way he's character one. Invert it and he becomes character two...
I still have some things up in the air, but I hope they will work themselves out as the story develops.
As for me, I'm going with the old fashioned approach: odd-shaped stone object, possibly brought to Earth by alien beings, which opens portal once properly assembled.