If I recall correctly they are not meant to be connected simultaneously in that there is no input switching available. The unit will always default to the USB input when both are connected, unless USB is fed power only and no data, at which point SPDIF is live.Can you have two devices connected at once to the Tone DAC? If you have one connected via USB and another via coax can it switch between them?
There is an I2C header on the board so in theory it could be controlled and switched, but I'm not sure how one would go about doing that.
I am referring to the original Tone board, or what is now called Tone1, I have no idea how the newer Tone2 Pro may differ.
EDIT: This post seems to indicate that SPDIF is live so long as USB is not actually playing. I can test that for you with my board.