If Hitler really thought such an alliance would go to war, he would not invade Poland.
Hitler thought that the only valid to go to war was lebensraum - to leave the people a greater homeland they could call his own. He dismissed wars fought for other purposes as senseless because they would not provide any benefits to the country. Indeed, such wars weakened the country by killings its able men. His thought on such matters are actually more sophisticated than many people think. Hitler in 1939 was not the Parkinson's deranged bitter madman he would be by 1945.
He certainly would not have entered a two front war. He only invaded the Soviet Union in 1941 because 1) it would have been the only real land war he was facing at the time, and 2) he did not think the Soviet Union would survive after 6 months.
Met with such obstacles, I think Hitler would explore other diplomatic options. He did want a short little victorious war to blood his armies and install some fighting spirit in his people. And he needed the plunder to keep the German economy going. But he never intended to start a major war in 1939 (he planned for one in 1942 or 43). And he certainly did not want a two front war.
I think Hitler would rely on diplomacy to splinter Britain & France from the USSR. He would probably hope that the presence of the Red Army in Poland would prove disruptive, and that some situation would arise fairly soon that he could divide his enemies and go to war against someone. But he would want that to be a limited war until Germany's rearming program was completed in 1942. Given the economic issues he would have between then and later, who knows if he could meet those goals.