So the stab is trimmed for 240 knots. 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.
When one column over rides they other, full control is given to the operable control column. the control surfaces are not split.
Again, this would be the case in older Boeings. Do not know about this one.