It's better if the additional power drop comes directly from the transformer. Depending on what transformer you have, you should either be able to add a second set of wires to the terminal outputs or split the existing wires into two - one for each power drop.
You could also hook your transformer output wires to a terminal block and then run two sets of wires to your track, but that's a little overkill for a 20' layout.
No need to use a second power supply.
