One highly possible outcome from a japanese war against russia is, Russia quickly defeats the japs in manchuria and korea. This would free large numbers of soviet troops to fight against the Germans.
So untrue...
1.) ground warfare, manschu is bad for japanese without heavy gear, korea on the other side is a nightmare for every tank army. Japanes army in korea would have binded 2-3 mio enemy forces, even when russia+china pushed together against 1941 japanes army. Ask USA UNO army during korea war, how 1mio chinese during perma bombing and no tanks could push them so hard in korea.
2.) numbers: 1941 soviet forces in far east were 720.000 men, from this number sovietunion transfered until 1942 600.000 men to western front against germany, 400.000 until dez 1941, which wer used mostly during the counterattack at moskau, which made up to 40% of all troop used during the battle of moskau on soviet side
the japanes imperial army in 1941 had 1.700.000 men, in 51 division, 27 divs were in china (900.000 men), 13div in manchu (430.000 men), the last 10 div (reserve of 330.000 men) were used on other fronts and home defend.
To counter the 1.300.000 million ground army of japanese, without china, in 1941 in far east, soviets had to dopple there far eastern army from 700.000 to 1.400.000....to understan this number, during 1942 when soviets fought stalingrad, to secure stalingrad soviets used 1.400.000 troops, the counteroffensive in moskau in 1941 used 1.000.000 troops. the failed attack in near moskau in 1942 against AG center used 1.400.000 troops?
where the hell should the soviet army get 1.400.000 more troops for the far east,. when they reduced there starting army from 1941 to 1942 from 700.000 to 200.000 in far east to help at western front?
3.) reserves: the total soviets army over the years from 1941-1945, compare to german army at western front
Date // German + allies // Soviet forces
June 1941 // 3,767,000 3,117,000 (German) 900,000 (Germans elsewhere) // 2,680,000 (in theater) 5,500,000 (overall) (~700,000 in Far East
[37])
June 1942 // 3,720,000 2,690,000 (German) (80% Germans in the East) // 5,313,000 (200.000 far east)
July 1943 // 3,933,000 3,483,000 (German) (63% Germans in the East) // 6,724,000 (700.000 fe)
June 1944 // 3,370,000 2,520,000 (German) (62% Germans in the East) // 6,425,000 (700.00 fe)
Jan. 1945 // 2,330,000 2,230,000 (German) (60% Germans in the East) // 6,532,000 (Soviet build up in Far East accelerated greatly since February
[37])
Apr 1945 // 1,960,000 // 6,410,000
Japanes army during this timeframe
1941 // 1.700.000
1945 // 5.500.000
So a eastern front around siberia would be the little to much for soviet union, they would need 2-3 mio troops in the east, to hold japanes in bay in korea, or they needed 1 mio troop to hold japanes in bay alongside of transibir railroad. which would reduce there numbers in 1941-1943 at western front so high, that all the sucessful counteroffensives the soviets did, historical, are hard to see that they wozld be sucessfull again.
Image the moral hit to soviets, UK and USa from dez 1941....when counteroffensive failed, becaus eof lack of troops or to little numbers.
Image 1942 stalingrad, when soviets could muster 1.400.000 at stalingrad, but not 1.400.000 at moskau at the same time, because this troops are in the far eas binded.
Image nordlicht, without soviet counteroffensive.
4.) Leand and lease, 50% of leand and lease were transfered through vladiwostok alongeside transibir railroad.
From august 1941 to dezember 1941 through USA shipping, from dez 1941 only thorugh soviet shipping. 8,244,000 tons, or 50% of all support over this way.
Persian corridor, which had 27% of all shipping, were 4,160,000 tons from mid 1942 (before it was not running)
Northern route, 23% of all shipping, 3.900.000 tons, running from aug 1941. loosing rates of 7% of all shipping thourgh german interruption on this route.
With japan in war, soviet union would only get 33% of all leand and lease from 1941-mid 1942....from mid 1942 they would get 50% of historiccal leand and lease.
5.) casualties military
Soviet union military deaths: 8,668,000 to 10,922,000 (only at there western front)
German military deaths: 4,440,000 to 5,318,000 (western + eastern front)
[
German allies: hungary 300.000, romania 300.000, italy 320.000, finland 83.000
means kill death rotation from roughly 1:2 for the soviets at there western front, so for every enemy they had to kill in military operation, they had to loose 1 more soldier than the enemy side.
Now look to japanes and china, the main ground for japanes war.
China: 3,000,000 to 3,750,000
Japan:2,100,000 to 2,566,000
[
Now the battle of Khalkhin Gol,
Soviet union: 28.000
japan: 23.000
So we can say, kill deat 1:1,2, so soviets need 20% more man to kill 1 japan.
Transfered to japanese looses in ww2, soviets had to pay 2.500.000 to 3.000.000 extra looses from 1941-1945 with a easteren front, and japanes would focus sovietunion and not china.
Facit: if japan and germany would be done a real alliance with a shared enemy and shared wargoals, and not a paper alliance with "we fight everybody and do not concentrate our forces against 1 (bully). I do not see, how sovietunion would have won a 2 front war, with the switch of numbers and logistical support,. they loose through this.