To add to the tandem pumping, I managed to try it when I was building up my milk bank before heading back to full-time work. Eva was around 8-9 mths at the time. It is different than pumping without her latching on - in a good way. Tandem pumping is often more effective because of the letdown that occurs at the same time when a baby latches on. Often some mummies will leak but imagine pumping during a letdown. If you're lucky, you can get two letdowns in one session, something I also experienced only with tandem pumping. This is useful for those with babies who only take the main course (read: feed from one boob) and/or wish to store up their milk banks.
But like you mentioned, this only works if your kiddo isn't distracted by the pump - those with silent "wireless" pumps might just find this easy to do.