They'd sit back and watch the two bleed each other white while rearming as rapidly as possible to deal with the weakened eventual winner if an excuse could be found.But why should France and Britain intervene on the Soviet side in a German-Soviet war? As much as the WAllies dislike Hitler, they are anticommunists to the core, so I don't think they would want to sabotage the destruction of the Soviet Union.
One thing no one brought up is that the Poles were rabidly anti Soviet and prior to 1939 are likely to join Germany in any war against them, along with the rest of Eastern Europe.