Double pump or 2 single pumps?

I would like to double pump as I heard it expresses more milk this way. However I see that with current promotions, 2 single pumps are cheaper than a double. Is it the same effect if I use 2 single pumps at the same time? Like the only difference I can think of is that I will need 2 plugs... But I have actually never seen a pump in action before so I dunno..