Optical takes priority on the D1, so if you had something like a computer hooked up USB and another device hooked up via optical, whenever the optical connection was active it would be automatically switched to that.  That means that, depending on how the second device's optical out behaves, it might automatically switch back to USB when nothings playing, or you might need to power down the device (or unplug it) to get USB back. If the second source's optical output does not shut off, one option would be to use an optical switch.  You could connect both the computer and other source to the optical switch, and the switch to the D1.  Then you'd have a easy way to see which was active and switch between them.