Young Grasshopper is right. Due to the turn sequence (Germany, then USA, then Italy) the US fleet would most likely destroy those German carriers so they would not be there for the Italian planes to use.
Now, here is a scenario where this might work. Since Germany and Japan go before USA, UK and ANZAC, any of those Allies would work here.
Say that Germany has punched though the Middle-East and captured Calcutta (India) and controls the now Minor IC. Next, USA, UK, ANZAC or even all three of them move warships in SZ 39. On Germany’s next turn, they put down 2 carriers in amongst the Allied fleet. Then on Japan’s turn, they fly out planes (fighters and/or tacs) based on Borneo (4 spaces away) to attack the Allied fleet in SZ 39 and land on the German carriers.
In this scenario, you are correct. The German carriers WOULD NOT be able to take any hits in this battle because they are not involved in the actual combat. Since the Japanese planes are attacking, only the Japanese planes are involved in the combat. The German carriers are effectively “neutral” since only 1 power’s units can be involved in an attack at one time.
Once the battle is over, assuming the Japanese are victorious, the planes sink the Allied ships and land on the German carriers. Then, if say the US had further warships in the area and attacked SZ 39, BOTH the Japanese planes and the German carriers would defend.