Yeah it seems pretty impossible but how much more impossible than being Mr. Robot? If I must accept that in every past scene with Christian Slater he was either Elliot or not there at all then I guess that I can accept that his scene with Tyrell was actually two of Elliot's personas in conversation. I can also accept that in Elliot's scene with Joanna Wellick that she spoke to him as a different person because she is aware of his duality (triality? quadality?) and was fishing for clues as to when he would become her husband again.
Whatever the truth, we know that there'll be plenty more mindfuckery next season.