You can't converse on J on CX. And please don't try to either, as you'll end up shouting and disturbing the other passengers.
In other words, if you want to converse during a long flight then don't fly cathay business class. If you really must fly Cathay, then F (sit together or opposite on ottoman assuming seatbelt sign isn't on) or Y are your options.
However, if you like sleeping in a coffin like environment and that is all you really want to do on a long flight, then Cathay is your perfect choice.