The distances involved meant that mutual cooperation was nearly impossible. More importantly, for the Allies there was little overlap in equipment and troops in each theatre. If they weren't fighting Japan, it is not likely the material assets from the Pacific would have made that much of a difference in the war, except perhaps in the form of aircraft carriers that might have helped defeat the uboat threat.

That said, there was one particularly important lack of cooperation between the two powers that definitely did hurt Germany at least, and that was the Japanese-Soviet Neutrality Pact of 1941. This was signed shortly before Operation Barbarossa began, and allowed the Soviets to redeploy 25 divisions from the Siberian frontier to the European front against Germany (http://www.operationbarbarossa.net/the-siberian-divisions-and-the-battle-for-moscow-in-1941-42/#Red%20Army Divisions Transferred West from June to July 1941). The effect these divisions had on the stabilization of the Soviet lines in late 1941 is debatable, but 25 divisions is not a small number. That said, experience prior to the war in brief skirmishes between Japanese and Soviet troops suggested that even the 1940/41 era Red Army was more than a match for their Japanese opponents, meaning that it's entirely possible that Stalin would have transferred those divisions anyways. Japan couldn't afford to divert too many units from the Chinese theatre as well, meaning any attack on the USSR would likely have been relatively weak as well. However, if Japan had decided to press on behalf of Germany in June/July 1941, they might have been able to hold back those 25 divisions, which may have had an impact on the Battle of Moscow.