The article doesn't mention SpaceX, I wonder what's the difference in energy and cost for launching (and landing) the relative stage 1s. Does anyone have an idea on that?
At separation, Falcon 9 first stage is doing about 2500 meters/second and is at about 70km altitude. The two connected 747s flying in formation don't get that fast (or that high, but horizontal velocity is much more important).
The Falcon 9 first stage releases stage 2 at around 6-10 times the height and 10x the speed.
That said, remember that the difficulty of elevating yourself above a given height is much worse the lower you are. Using a plane-drop means skipping the very worst parts of the launch. The air is so much thinner.
There's other advantages though. Launch windows get much easier, as the plane can fly in any direction, and stay at a given heading a bit longer.
