There is no point, unless one can be found cheaper or used, it is because they are both made on the same Die(Board/Mechanical make-up), this is the same with every AMD GPU.
A HD 6950 and a HD 6970 can be CF'd
A HD 5750 and 5770 can be CF'd.
A HD 4750 and 4770 can be CF'd.
A HD 3850 and HD 3870 can be CF'd
But you will only get the performance of two of the lowest clocked card.
Before you ask question at least read the FAQ's and/or Stickies.http://www.tomshardware.co.uk/forum/245454-15-crossfire-faqs