And behold! We are back where we started. To send US forces down to Australia or not to send? Ti send the whole fleet or not to send? I still say that I like having America in two places. Earlier posts have proved that Japan cannot ignore either the north or the south without difficulty. The best Allied offensive maneuvers rely on mobility, calling your enemy out of his armor, and not allowing the Axis to focus it all in one place.
The main argument for splitting up an American task force is that Japan typically has to deal with both halves.
Japan does not have the resources, units, or incentive to go after you on both fronts. Just sit back and keep the imperialists sweating because they are spread too far as a result of your threats.
I have three ways of cracking the nut. My personal favorite, a smaller aerial and low-cost naval force in Australia to counterattack Japanese counterattacks in the Dutch East Indies. Meanwhile, a larger American navy up north is preventing an equal portion of the IJN and its airforce from moving anywhere. Japan eventually gets stretched too far and forced to spend more IPCs on the Pacific than Asia.
Another option: the historical approach of hitting Japan at its Pacific core. The obvious place to go is the Caroline Islands where you can threaten so many places at once. Iwo Jima, New Guinea, and others can be used as well. The problem with this strategy is that it requires quite a bit of spending in order to make keep an American fleet safe in the heat of the Japanese forces.
And the last option: the late-war method of subs and bombers. Spam the Pacific with so many submarines that Japan is getting convoyed uncontrollably and afraid to push around its transports. Meanwhile, use Russia as a springboard to bomb Japan and Chinese factories as well as get a land war started up north.
These are just a few ways you can hurt Japan. There are many other great Allied strategies such as the famous Alaskan/Aleutian shuttle line and the lesser-known “Fortress Singapore.” But nearly all strategies require distraction, pressure, and keeping your cool. The point is this: don’t just stack all your American navy in one place. Unless Japan has become inferior in their naval assets or unable to spend on ships, you’ll just spend the whole game doing nothing (unless you manage to get deep enough into the Pacific that Japan cannot rely on blocking you with a single navy in Japan/Philippines/etc.
Hopes this helps you control that beast of a nation.