The two MIDI inputs allow connecting a MIDI controller (keyboard, drum pads) to one input while the other input is used to receive MIDI Sync from another unit such as a digital recorder. Otherwise you would need an outboard MIDI Merger to accomplish this sort of setup.

The two MIDI ouputs allow the MPC1000 to control 32 external MIDI channels (2 outputs, 16 channels per output) in adddition to the 24 internal Programs. This is very useful if you have more than one outboard multi-timbral sound generator that you want to control or send MIDI data to (ie, a keyboard and a sound module).