Soon after the initiation of Operation Barbarossa, debate began within Japan concerning whether they should launch an attack to the "North". Foreign Minister Matsuoka was a chief proponent (even after claiming the recent USSR-Japan non-aggression pact as a personal triumph), believing that Germany would quickly crush the Soviets and Japan would get nothing unless they joined the assault.
The Navy opined that it was not possible to attack "North" and "South" at the same time, and delaying the southern attack was debated. It appears the Japanese thought US intervention in response to an attack on the USSR was unlikely, but "possible" (but also believed the US was not yet prepared for war).
Berlin made a formal request through diplomatic channels on 30 June 1941 for Japan to initiate a second front.
So, what happens if Japan delays their attack against Singapore, the DEI and the Philippines while launching an attack against the USSR?