We have done quite a bit of work on "twin lift" with good results, but the needs to engineer solutions to the obvious problems (Obvious, one would think, until reading the news report!).
Dynamic interactions between machines, controls issues as they both trim to 30 degrees of bank away from each other, control issues as to who is flying and who is monitoring, etc.
The solutions involve coupling both flight control systems via electronic link, use of a rigid beam between the aircraft, making one aircraft the master and one the slave, and reducing maneuvers to the basics.
Done otherwise, such operations cause "massive noise complaints."
The Piasecki "Helistat" destroyed itself with dynamic interactions (like a low frequency flutter). It was a composite of 4 helicopters and a gas bag.