FO pushes full forward. Captain pulls back and splits the yokes (as designed). So you have a stab trimmed for 240 knots, one elevator full down and one elevator full up. And a lot of power has been added, which of course is "nose up".
That does not result in what the airplane did.
Note that the split elevator system on the 767-300 has a limit of 20 degrees between each side. There is a torque tube in the tail tying the two sides together, but there is an override mechanism on this torque tube which allows independent left/right side movement up to 20 degrees. The column force required to activate this override is 25 lbs at breakout, increasing to 41 lbs at 20° of elevator movement. This is related to the tail stuff. The override at the front (between the columns) is also 25lbs. With no air load, the elevator can move a maximum of 28.5° up and 20.5° down with a full forward and aft movement of the control column. If the autopilot was engaged, they would be fighting the camout springs in the elevator servos (until a camout was detected and the A/P disconnected). On top of all this, you have elevator feel.
You'd need to be a rocket scientist to figure out the cumulative effect of all these devices.
Shear pins? Que?