Can I send audio to both the optical and USB inputs at the same time?

Unfortunately that is not how the D1 is designed, as the optical connection will take priority. You may be able to have the optical and USB connected to different devices, but it depends on the how the optical device operates. Some devices will deactivate their optical out when no media is playing. Others will leave it active and send an empty signal when no audio is playing, in that case you will want to shut the optical source off. It can be bypassed when using something like an optical switcher so that you can toggle between the optical source and the USB input.