Since both burned pistons were at the back of the motor, it's possible it was a fuel distribution problem causing those cylinders to run lean. Could also be an air/vacuum leak. What fuel system are you running? How did the plugs and combustion chambers of the other cylinders look?

How is the motor used? Did the blow by come on suddenly or was it over time? I guess I am asking if it could be tied to a single event or usage.

Pat