I've been thinking a bit more about this and I can see why some might think that the 400+200 might mean a 600 has been done, after all 100+100 means a 200 has been done. It'll be well confusing in the points lists though if the 400 is not going to be counted twice.
A 200 and 400 ridden in this way, in my opinion, does count as a 600. They are not two seporate rides, they are one ride made up of two.
Mike Kelly still had the time allowance for a 600 when he rode the 200 and 400. He still could have been timed out and couldn't decide to ride the permanent part of the ride later in the day after some rest.
The only reason we split the rides up is for points counting purposes. When this was done before as a DIY, we counted the total distance of the DIY as a permanent. I believe that if we never had the 50% rule, then these rides would count as total distance and not two seporate rides.
I think it is important to acknowledge the rides for their total distance. Not so much for points chasers, but more for anyone aiming for an SR or their longest ride.
After all, AUK voted in favour of maintaining the 50% rule, so it would make sense to encourage people to ride calendar events and this seems like a good way of doing it, by padding them out into longer rides.
Also, if you think that ridng a 200 and 400 count as two seporate events, where do you stand on the time allowance if you ride a 300 and 400 where you would have a slower minimum speed of 13.3kmh if it was a 700, but not if it is two seporate events. Or would you allow a rest period. It would make things much more complicated.