Kitty2 - How does that work then?
If they have to turn in what you gave them (say you gave $200) and then they can keep what extra you gave them (say $100). You could reverse this and give $100 as the 'main' tip and their personal top up of $200.
It could be argued that the larger amount has to be given as the main tip. People could still get around this by giving $100 as the main tip followed by three seperate tips of $99 to the person on threee occasions. Or people could just add it up to $300 and give $149 to the person and $151 as the main tip.
Personally, I think what you as a passenger choose to give as a personal tip should remain as such.