Unlikely bordering on impossible. The bulk of US forces fought in the eastern pacific as part of the 'island hopping' campaign, while most of the Russian forces fought on the Asian content. The Soviets actually carried out very few major operations in the pacific as the Nazis were beating them up pretty badly on the European front. Gen. Stillwell did have some American forces in China but compared to the ETO they were pretty minor. If you wanted to have Soviets and US interacting in the pacific the most likely case would be an airmen whose bomber was damaged and landed in Russia after being unable to make it back to base. To my knowledge the US and the Soviets didn't carry out any joint operations in the pacific and were generally separated by thousands of miles in their operations. The Soviets were only our allies in the war in the loosest sense.

Possible, but only towards the very end of the war. Japan and the USSR signed a five year non-aggression pact in April 1941, which both sides observed scrupulously until the USSR declared war on Japan on August 8th 1945. At that point the Soviets began overrunning Japanese forces in Manchuria. You might conceivably have had an American military observer with those Soviet forces, although that's pretty unlikely. Crewmen from a B-29 forced to divert to Soviet territory would be another possibility.