My view, be it correct or not, was that both players tried to stop Sterling by getting in his way as per Piltdown Mans explanation of the old obstruction lawn BUT when they both do it together this means that he was sandwiched between the two so a foul was committed.
As the pundits said they both contributed to it and 1/2 + 1/2 = 1 penalty. Either defender on their own and it would have been obstruction but together it is a foul.