The USB protocol always works point-to-point, i.e., it is not possible to have two USB devices that, from the point of view of the computer, behave as if they were a single device.
You have to use multiple USB/MIDI interfaces, or a single USB/MIDI interface with multiple ports, or a separate MIDI splitter device.
Some big interfaces like MOTU Midi Express XT/Micro Express and many iConnectivity devices, do allow you to customize the routing.