you should look into the Unitor interfaces, also the MOTU Midi Time piece AVs. they have the ability to network together, which in addition to making management and configuration from the computer easier, also allows you to set up routing matricies that go between units, so you could have your master keyboard on port 1 of interface 1 playing port 7 of interface 2. its great if you're working with a lot of rack-mount modules like I do.
(edit) the older opcode interfaces did this too, but, good luck finding one of their USB versions.