Yeah I would have to dig deeper into it to find out why he does it that way.
He does it that way because he wasn't taught properly.
The main application for the double Tok Sau is its use against an opponents high guard.
The opponents forearms are too high to properly use a Pak Sau to break through so the double Tok Sau's are used to come underneath the opponents elbows which off balance the opponent and opens up the defence.
The way Benny Meng does it is wrong , visualize a big strong dude holding his arms up rigidly , Benny Meng is just basically getting underneath and trying to lift up the big dudes body weight.
You will not move somebody like that.
Chum Kiu theory tells us that we should be attacking with multiple force vectors at the same time , it is very difficult for an opponent to defend against more than one force vector.
So by doing the Tok Sau correctly by going up at an angle towards the opponent you are using two force vectors at once , both up and forward.
This has the effect of unbalancing the opponent and sending him back rather than using Mengs version which is basically just trying to use strength to lift the guys arms up.
In our lineage when we do this against an opponent with a high guard we also use it with a thrust kick.
The double Tok Sau's open them up and off balances them , and the thrust kick sends them on their way.