As we know, Nazi Germany and the Soviet Union amicably agreed to attack Poland and to share it between themselves.
Did Hitler and his staff have a detailed or formal contingency plan in case the Nazi-Soviet talks failed and no agreement was reached? Would they have gone on with the invasion of Poland on their own anyway?
I am asking not for counterfactuals but for documented evidence of either contingency plans of staff work on such plans.
One answer would be that a military "contingency plan" of sorts was written into Fall Weiß itself - the operational plan for the invasion of Poland was written so as to begin no later than September 1st, 1939. However, it is probably very unlikely that the invasion of Poland would have been canceled if Treaty of Non-aggression hadn't been signed prior to that date. In particular, it was fundamental to the military planning that the invasion attain surprise and conclude quickly to prevent the full mobilization of (and thus allow the destruction of) the Polish military.
The operational plans of Fall Weiß were finalized on June 14th, 1939 and deployments were already underway before Stalin's meeting with Ribbentrop on August 22nd, but to take the Soviet pact in isolation ignores the fact that the diplomatic situation at the time was incredibly fluid and the non-aggression agreement was only one part of a much larger puzzle.
There were two primary objectives that were being pursued prior to launching the invasion of Poland. The first was to ensure that the Soviets would not interfere with a German invasion. However this would have been seen as unlikely given the historical animosity between Russians and Poles, which was evidenced by the refusal of the Polish government to even consider an agreement that would allow the Soviet military onto Polish soil to resist a German invasion. In fact, this is one of the larger difficulties that the French and British were having over the course of the summer of 1939 in their diplomacy. Also, keep in mind that the Germans were well aware that the Soviet military had already been engaged with the Japanese in the east since May.
Second was to ensure that Germany would not be attacked from the West while they were occupied in Poland. This meant having a reasonable level of confidence that the French and British would not attack from the West in the aid of the Poles. At this point, the only planning on the western front was military posturing along the Siegfried Line and targeted propaganda in the media with the intent of causing the French to over-estimate German military strength in the West. The military planning for a war with France (Fall Gelb) didn't even have a first draft until October 19th.
So from another standpoint it could be said that delaying the scheduled August 26th invasion of Poland for a week after the signing of the Polish-British Common Defense Pact was in itself a "contingency plan", allowing the German military time to start a more general mobilization and make a last ditch effort to preserve the possibility of some sort of agreement with the British. But even when these talks with the British broke down, Fall Weiß was executed on its September 1st deadline with every likelihood that it would lead to a declaration of war from both the British and the French.
I have never seen any reference to a military operational plan for a two front war that existed in 1939, and I seriously doubt that there was one. The fact that Hitler basically dismissed this and ignored the Common Defense Pact would seem to support @Kunikov's answer that Hitler would have proceeded regardless of the Molotov–Ribbentrop Pact.
First, the Soviet Union and Germany never agreed to attack Poland or share it. They agreed on spheres of influence, which would come into play with 'border changes'. Said 'border changes' did not mean an invasion. Hitler had accomplished much without war and there was little reason for him to think he would not get away with victimizing Poland with no real consequences from the western allies, especially if he could get a pact going with the Soviet Union.
Secondly, the Soviet Union, or more specifically Stalin, had no idea that Hitler would be invading Poland. They waited to see what would happen knowing that if territory did start getting detached and passed around, they would get theirs (similar to how Poland received a piece of Czechoslovakia).
Now, in the case of no non-aggression pact with the Soviet Union, German plans for an invasion of Poland were already 'set in stone'. The invasion would have commenced with or without Soviet 'cooperation'. Hitler was a gambler, he would have gone ahead with the invasion betting that the western allies would do nothing of worth against Germany.
Hitler was not in the practice of making "detailed contingency plans". He was more of "failure is not an option" type guy. If you read Speer's book, "Inside the Third Reich" you will find that it makes clear Hitler considered the pact with the Soviet Union an essential precursor to attacking Poland.
The strategic situation demanded this for several reasons. First of all, at that time Prussia was isolated from Germany proper. If Hitler invaded Poland, and England and the Soviet Union declared war, Prussia would be completely vulnerable and Hitler would be facing the situation of having 20% of his own nation immediately occupied by the Russians, something he would have considered completely unacceptable from a political standpoint.
The other strategic consideration involved was a miscalculation. Hitler reasoned that if he was allied with the Soviet Union, England and France would not dare to start a war against him. This opinion proved to be incorrect.
