You can only connect one modem/router to one line with NBN. If you start connecting other devices, it'll interfere with your service and you may not be able to use it.
What are you hoping to achieve here? An additional phone in a different location? A separate computer in another room cabled to your main router?
If you can give us an idea of what you're trying to do with your second modem/router idea, we can likely give a more meaningful reply