On looking at it, it appears that using the softkeys to record or play operates both samplers, so it records or plays all (4) channels.

If you use the inputs 1 and 3 to perform an audio triggered record or play, you can drive the samplers independently.

The inputs 1 and 3 refer to the inputs to the preset, after the routing. These will be DSPA_IN1 and DSPA_IN3.

The INFO page correctly describes the operation.

Note that the trigger EQ allows more control over the audio triggering, for example you could set it up so that a bass drum triggered the record or play.