I expect there are going to be several different opinions on this, so it probably deserves its own topic.
Looking at the forces that a twintip experiences, there is never going to be just one answer as the loads are dynamic in nature.
However, I'm going to be looking at two specific scenarios and try and explain my thought processes.
An important assumption is that the board can be treated as an ideal beam. Given that there is a fair amount of precedence for treating ideal beam analysis as a fair representation of real world beam behaviour, I do believe this assumption is valid. We can of course debate what are suitable values for moment of inertia and elastic modulus, but certainly the qualitative results will remain the same.
The first scenario I want to analyse is where the rider's weight is entirely supported by the water load (pressure), with no velocity relative to the water. This is of course an idealised scenario, and the closest real-life instance might be where you land flat on the water coming down from a tiny little jump, at zero forward velocity.
Anyway, the reason this is useful is because then we can approximate the load on the board as a simply supported beam with double overhangs, experiencing uniform load. Of course, in our case the diagram is turned upside down, and the reactive forces are the rider's feet.
So what the red deflection line tells us is that the tips as well as the centre of the board experiences an upward deflection, and the bits under the rider's feet experience a downward deflection.
Also, in this state, the board is the most evenly loaded and all parts of the board contribute towards supporting the reactive forces (the rider's feet).
Any other scenario where the loading is not uniform anymore will mean that some parts of the board will need to work harder to support the load. We can look at that next.