Same answer, same reasons
But I can give you more detail with that info.
When you click "Send to Device", a DeviceJob is queued up. That DeviceJob calls a specific function that all drivers are required to implement which handles sending the book(s) to the connected device. The extended Kobo driver hooks into this method of the mainline Kobo driver (if you know some object-oriented coding, technically KoboTouchExtended extends KoboTouch, implements the appropriate method, and calls the superclass method) and so only one send-to-device conversion may run at any time.
What you may consider is using the KePub Output plugin. It allows you to convert books in your calibre library to KePub format, and multiple conversions can be done at once that way. The KoboTouchExtended driver is aware of this and will skip the conversion process for .kepub books in calibre. Just make sure you enable kepub as a format in the extended driver options (the big list at the too of the settings window) and that kepub is higher than epub in the list.