I'm not convinced this doesn't verge on ASB territory, given the nature of the alliances at this point--although given the volatility of Wilhelm II, it can't be discounted out of hand, no matter how remote the possibility.
Thus, setting aside the question of likelihood for the sake of playing the game, it seems to me this conflict would be a tragic comedy of errors, given the relative ineptitude of the military "organizations" (using the term extremely loosely) of both sides. Battles would be "won" by whichever side managed to avoid more egregious blunders than the other, but for the most part, it would be a chronicle of screw-ups, thick-headedness, and downright incompetence on both sides.
It would come down to this: which collapses first? Would it be the Russian side with its corruption-riddled supply lines and inadequate manufacturing infrastructure, or the A-H side with its polyglot ranks and strong probabilities of orders misinterpreted / misunderstood? I'm guessing after about eighteen months of both sides suffering significant reverses, a draw would have been declared out of sheer exhaustion. It would have been so much sound and fury accomplishing zero, except the acceleration of a day of reckoning for both rather rickety empires. Probably the outcome is a status quo antebellum peace for the big boys with Serbia under the thumbs of both powers: A-H as a form of revenge for regicide; Russia to make sure the locals don't do anything quite that stupid any time soon.