This is not the first time when Jews were found on two sides – there were Jews in Austrian and Russian armies during WW1 (aka Great War). There were reports of Jewish soldiers seeing tzitzis on the niftarim and wounded in the enemy trenches.
In the case of Jews in the Russian army, if they are there against their will, maybe we should include them into tefilos for shevuyim? If they are there voluntarily, then maybe honen hadaas, ask Hashem to give them sechel, and if they are real reshayim then in minim. I doubt, though, that there are many – It seems that most of russian army recruits are contractors from prisons and remote areas.