香港如何加持第二本護照?

There is no explicit legal authority authorizing or prohibiting the Immigration Department (in the name of the Director of Immigration) from issuing the second passport. So it is possible that the Department can issue one under its discretionary authority. However, it is also possible that the Department will simply replace your current passport instead.

You will have to actually apply for one with the Department and see how the Department handles it. A compelling reason with proof is always required.

However - unless it is extremely necessary, it is a worldwide practice avoiding the issuance of second passports, even explicitly allowed.