A follow up to your question, if I may... When did the White Russians dissolve as a political and military force? This battle here was in 1934. I was under the impression the Whites had basically been dissolved after the Civil War, back in 1924ish.

We're the Whites of 1934 still advocating a return to aristocracy and the Tsar?

There was a significant population of Russians living outside of Russia following the revolution as part of the White émigré following the change to the new Soviet government. While many left following the command of the Bolsheviks, others left fleeing the later Soviet government and are often (and arguably incorrectly) lumped into the term "whites" as a result. These expats included Russian imperial officers and men, Cossack, wealthy individuals or land owners, or purely people fleeing the fighting. Many of them ended up in China, with major Russian populations in several areas.

I cant find any solid proof as to why the Whites may have fought alongside the Soviet forces. However, I don't think its a case of the Whites and Reds agreeing and going after Chinese forces, but rather the Reds and Whites fighting the same foe. The Chinese may have simply attacked the White Russians, fearing them to be part of the Soviet forces, or the Whites assaulting the Chinese fearing an attack.

In addition, it may have been a chance for Whites to gain leverage on the reds by attacking and driving the Chinese off and taking Soviet POWs or intelligence from officers. Its logical to assume, too, that the Whites may have come to aid Russian brothers purely because of ethnic similarity.

TL;DR It might have been a case of mutual hate, and they never fought directly with on another but against a common foe on two different fronts for different reasons.