**It must be difficult to practice a stunt like this.**
I didn’t read the story, but I assume for practice they had a co-pilot sitting in the left seat, in the left plane; and a co-pilot sitting in the right seat, in the right plane, ready to fly the planes, if the thrill seekers had failed to reach each other’s plane by a designated alttitude.
They probably pulled the doors off both sides; and for the big stunt pulled the other front seat out, since there would be no co-pilot.
Engine off, but master on, autopilot slaved to nav point, with a decent, and glidepath parallel to the other plane.
Turn off the master, and try it. :)
IIRC, there has already been someone who jumped from one piloted plane to another.