hardware

USB/MIDI cable

There are relatively inexpensive devices that make a 1-to-1 transition between the two hardware standards.

Midi Merge device

i.e. (link), is a device that merges two MIDI cables into one.

ESI Midi Mate II (link)

Is a bi-directional USB/MIDI cable with two MIDI connectors. This makes it a possible merge interface for connecting two drum modules (MIDI OUT) to one USB port.