Thinking about it a bit more...
With the two trains, do you intend to have them both running at once, or do you simply want two trains to be on the layout?
In the second case (two trains, but only one moving at a time) you can get away with one powerpack, and "strategically" located sections of track that you can turn off and on with simple electrical switches.
For example - in the "upper yard" you could have on-off switches for each of the tracks. A train pulls into one. You turn off the power to it. Turn on the power to the other track and pull out the loco that was sitting there. Simple - two trains, one powerpack, however many on-off switches.
To answer your question about points (turnouts) controlling the power - this is a good application for power-routing turnouts. Those that are reliable send power only to the route they are aligned for. They do not really act as a selector for multiple powerpacks.
Should I keep going? It may be easier for you to find a book on wiring. If you stick with DC control, Atlas has some good simple books that explain how to use their brand components in a variety of situations. Makes things very simple to set up.
Andrew