I think Japan must attack the US at Pearl. It is far more dangerous to leave the US fleet in Pearl than the UK fleet in Sz 30.
Some players will sink both, but there are pros and cons for that on both sides.
If I’m Japan, I’ll usually leave that merged fleet and deal with it later if I have to with the full J Navy/airforce. I could run into trouble if the Allies do go Pac strat, but I prefer to limit my J1 attacks and attack heavy (such as sending everything I can to Pearl and everything else to China).