There are a variety of reasons the Japanese didn't open a second front in Siberia. The Japanese had fought the soviets before in a series of border clashes but had been decisively defeated in the Battle of Khalkhin Gol by the heavy soviet forces and their relative abundance of artillery and armor. The Japanese didn't believe they were capable of successfully opening a second front and there were very large Soviet forces throughout the entire war positioned to guard against a possible Japanese attack. In addition, the Japanese army was stretched thin as it was with the war in China. Japan lacked the heavy forces necessary for such a war and their forces were light on tanks and artillery. In a ground war with a power such as the soviets they would have been very hard pressed even on the defensive as shown later in Manchuria. Overall, the Soviets were too powerful for the Japanese to engage and the Japanese armed forces were spread too thin to engage in another front.