Multi-device MIDI translator?

I have several MIDI instruments that transmit on the same channel (1). I like to build a device to input 2 USB MIDI input streams from separate instruments, modify the data (e.g. change channel number or note values), and output a result MIDI out stream via standard MIDI connector. I understand the Mega 2560 has 4 Serial ports that should allow me to do this, with suitable additional USB MIDI converters (8U2 flashed for MIDI?) or MIDI connectors. Is this feasible? Can anyone suggest suitable USB MIDI converters (type A socket) or MIDI OUT connector boards or shields (or MIDI USB out type B socket) that can use the additional Serial pins of the Mega2560 and not interfere with other Mega functions? Is there any sample code showing how to use these interfaces?