A second German sub in the Atlantic does pose a problem to the Allied fleet, but I would rather let that German sub do its business against the US and UK, and keep the Russian fighter to trade territories with Germany, than to attack the German sub and lose a Russian fighter to the German counterattack.
sorry to interrupt this thread with a noob question, but I thought subs could only attack other sea vessels, are you saying that it gets a counterattack against an attacking fighter?