In the FireWire audio standard, there can only be one “master” controlling a specific audio interface at any given time, either the host computer or the Paca(rana).
Some audio interfaces (like MOTU, Apogee, and the newer Behringer interfaces) let you switch “masters” relatively easily, using Audio MIDI Setup on the Mac, or the Device Manager on Windows, so you can be using the audio interface with either Kyma or the host computer audio software (but not both simultaneously).
Instructions for switching between the source of control for the MOTU Ultrallite can be found here: http://www.symbolicsound.com/Learn/MotuMacPacaAlternating
If you would like to hear audio from both the computer and Kyma at the same time, you could dedicate one audio interface to the Paca(rana) and another audio interface to the computer. With the FCA610, we would suggest that you use the Duet as the Mac’s interface and the FCA610 as the Paca(rana)’s, then connect the Duet’s output to the front panel analog inputs of the FCA610 (you may need to select the Instrument and PAD buttons and adjust the INPUT levels to make sure that the Duet’s audio does not clip at the FCA610’s input). Then you can use the MIX BAL control on the 610 front panel to control the audio mix between the audio from the computer and the audio from Kyma and the MAIN VOL control to control the overall level.