You would just end up with a mess that wont last any longer - colour fades each time you wash it and only lasts for as long as your new hair growth takes to come through, so mixing two colours wont slow down the speed of hair growth and it would still fade when you wash it
I'm told by my hairdresser mother in law that you would end up with a shade half way between both, but you should only consider this if you are using the same brand as different brands may not mix very well. Louise is right about duration - they won't last any longer mixed than they would separate. Overall I reckon you would be better off not mixing them though.... If you want something specialised get a hairdresser to do it.