Same case. I am using Hang Seng Integrated Account (Saving + Current). I called service hotline and they confirmed me my Saving Account is my default account.
Because I only have one bank account and one ATM card, in order to make sure I can get money at Taiwan, I plan to transfer 10K to my current account. So no matter which account shows up at Taiwan ATM, I can still get money.
I remember one mommy mentioned above that current account is her default account, end up she can't get any money. She said some ATM machines at Taiwan won't allow you to select which account to withdraw money.