Alienware m17x r2 gpu

hi. i been wanting to purchase the alienware m17x but with both gpu's (ati radeon hd 5780m) in crossfire its abit too expensive   so i was thinking to buy one and add the other manually later, but when i checked the manual, it does not say how to add gpu. so im wondering can it be manually added without causing problems? + id like to know what needs to be done to add the second gpu.