Unfortunately, this does not work.
There is no way in the 6EX to set the % on a master and slave seperately.
You can set the mix to +100%, but this only allows the 2 servos to work together in one direction (up). When you try to activate down elevator, only one servo moves.
BTW - the embedded link that you posted is from a 6X transmitter which is very different from the 6EX. Any chance you have different info for the 6EX?