Let me rephrase: The western front would have opened either way, but the War on Two Fronts situation would have been radically diminished if the Nazis invaded the Soviets earlier, as it was openly the plan of western reactionaries to pit the Nazis against the Soviets and then deal with whoever won after (hence their resistance to an antifascist alliance). The Pact (1939) was useful in stalling for time in an absolute sense, certainly, but it also helped prevent the Soviets from being effectively the sole focus of the Nazis (who, in the meantime and in their need for expansion, invaded other countries, including Britain in 1940), as Operation Barbarossa only started in 1941 (still earlier than the Soviets planned, but a delay nonetheless).

So my point is that the strain of handling these conflicts simultaneously was one of the major factors in the Nazis being defeated, but westerners were actively planning on preventing such a situation from arising, hence a need for the Pact to stall the Nazis (along with the more broad use of industrial development, etc.)