I have found a better solution. In short, the way to achieve my goal is to decalre two devices in hid descriptor: one for keyboard/mouse, one for a hid-data device. LibUsb can claim the device(s) even so. It works fine under Linux or Windows platform. If you want more detail, the website below is worth reading.
http://www.frank-zhao.com/cache/hid_tutorial_1.php
http://www.frank-zhao.com/cache/hid_tutorial_1.php