The easiest method would be to add a second MOTU interface. The 424 card supports up to 4. Similarly, FireWire users can daisy-chain multiple 828′s or 896′s, or hook up multiple interfaces to a FireWire hub for more I/O. DP and Audio Desk support multiple interfaces hooked up in this manner. If you use another sequencer, check with the manufacturer to see of they support multiple interfaces.
If your audio interface has ADAT lightpipe connections, another option is to add a dedicated A/D converter with lightpipe outs, such as the MOTU 8pre, to your MOTU interface for more I/O.