Yeah, not going to happen.
This is an over,kill but explains why viewtopic.php?f=8&t=11102
The v-usb consumes the entire CPU to handle NAK until the driver is ready. You will have no choice but to wait for it to do its thing before sending out. Simultaneous is not achievable but you could allow time slots with usbDisableAllRequests / usbEnableAllRequests
The problem is there is no way to predict the input data time to complete, it could be seconds. One trick you could do is skip the output until complete by adding flags. I have done that with great success but the window (normally 8 or 10 ms) for your bulk data will not be reliable.