The easiest method would be to add a second MOTU interface. The 424 card supports up to four. Similarly, FireWire users can daisy chain multiple FireWire interfaces, or hook multiple interfaces to a FireWire hub for more I/O. Digital Performer and Audio Desk support multiple interfaces hooked up in this manner. If you use another sequencer, check with the maker to see if they support multiple interfaces, or use an aggregate device.

If your audio interface has ADAT lightpipe connections another option is to add a dedicated A/D converter with lightipe outs, such as the MOTU 8pre, to your MOTU interface for more I/O.