Run your 18 INF back to Moscow as fast as you can at game start… You don’t have enough 2/3/4 attack strength units as the Soviets to stop the Germans if they are serious about Barabarossa from G1 builds on. You’ll be glad you did when it comes down to the major battle for Moscow on G6/7, history repeating itself for the win. Losing a few IPCs from the Japs doesn’t break the USSR immediately.
If Hitler decides to go for Sealion (or gets diced in E Poland :-D), stop, and wait and see what the Japs are doing after a few territories inland. I usually make a decision to counterattack or not around Yen or Tim if the Japs are threatening either through China or Siberia (though earlier counterattacks work if you can send a few planes to help). Commiting a few offensive units to the stacks helps in spades, why its best to wait. You can even help out in China and get your 2x stacking action with the Chinese.