The Russians did take pressure off the Western Allies with an offensive, just not with Japan. The Russians still had a neutrality pact with Japan after their border skirmish in Mongolia back in 1939. The Allies all agreed to deal with Germany first, and at the Tehran conference, Stalin agreed on a Russian offensive sometime in the summer of 1944. This would be Operation Bagration and it would blow open the entire Eastern Front and the gains from it would put Germany on the back foot for the rest of the war.

There is no strategic gain if the Russians attack the Japanese from the Far East. Why would this draw any attention for the Germans to send aid to Japan? Germany already had enough to deal with at the time, not to mention a non existent surface fleet couldn’t be sent with troops to support the Japanese. The problem at Normandy was a logistical bottleneck and the defensive hedgerows and dug in Germans that hampered the Allies slow expansion from the beachhead. It’s easier to fight a defensive war than an offensive beach landing.

Operation Bagration would commence on June 22nd 1944 and the Russians would attack Army Group Center. With 2.4-2.5 million men and tens of thousands of tanks and other armored vehicles, they would destroy Army Group Center in first few weeks of fighting. With the result of the end of the campaign being 450,000 Germans dead or captured of Army Group Center and around 300,000 of Army Group North completely cut off from the rest of Germany. Army Group North would be basically an armed POW camp for the rest of the war and would have no further impact in the war, Army Group Center and Germany in general would never recover from this. Combined with the later Falaise pocket being sealed and Germany losing France, the massive loss in material and men were staggering. And the advances into Romania and the oil fields by the Russians would also be a disaster for the Germans.

While attacking Japan would later be influential in ending the war for good, attacking them in 1944 made no sense. Russia already had a plan to support the West with their offensive. And in terms of success the Russians destroyed one Army Group and sealed another one off for the rest of the war. This opened the door to invade Germany and would also take pressure off the West; with reinforcements that could be used to hold up the West, instead would be used to plug the many gaps on the Eastern Front.

Edit: made a huge error, it was not July 22nd 1944, it was June 22nd 1944 the 3rd anniversary of the invasion of Russia.