Yes, you can simultaneously record inputs and outputs to USB.Remember you can record a maximum of 32 sources to USB. If you want to record your Main LR, you still can record 30 channels.
Go to I/O, where, click on USB in the top row (these are your outputs). On the left side of the screen you can find tielines, outputs and inputs. Simply patch here, what you want to record.
Hope that helps
André