Hi,
A simple way would be to apply the "Opacity" effect. Do this to the tracks which are using the "Track Motion", making sure that both have the same opacity level when draging the corners.
Then add another track and again use the "opacity" effect, and play with the levels and position of the new track untill you are happy.
Yes you can limit a track motion on a track. You do this by using the timeline at the bottom of the track motion tool.
Double click where you would like the effect to take place, you can use the time bar or the preview window to place effects.
Play with this tool and you will have some great effects.
I've added another short flash movie to show you how this works.
http://www.learningvegas.com/public/100.cfm
I think the suggestion earlier would work better for what you want.
Gordon
www.learningvegas.com