Yes, If only one source is turned on, you CAN leave both inputs hooked up. The DAC will automatically 'find' the working source and decode that data stream.
If BOTH sources are turned on, your DAC will either default to one, OR, just stay with the last one it was decoding (because it will not need to check the other.. depends on the internal logic)
If BOTH sources are turned on, your DAC will either default to one, OR, just stay with the last one it was decoding (because it will not need to check the other.. depends on the internal logic)