Steve,
You're in the right place. We have answers. Thanks to @RickO for starting our effort to provide them.
Here's my take:
1.) It is a good course of action because your preparing for the future, which you've missed about 20 years of actually, when you upgrade in this way.
2.) The Legacy remote will indeed operate conventional engines, but it needs additional help to do so. With a Command Control locomotive you can think of the the engine's "throttle" as being inside it. When you turn the big red knob on the remote it sends a command to the locomotive to move that throttle correspondingly. The voltage applied to the track is constant regardless of the position of the red knob, it is usually set at about 18 VAC, and it's not fiddled with after that initial setting.
This is not the situation with conventional locomotives. As has been the case from day one the throttle in these is operated by turning up, or down, the voltage applied to the track. So the additional help I mentioned is needed. It is in the form of a box that varies the track voltage in response to turning the big red knob.
These boxes are called PowerMasters. There are several choices for them, mostly based on power capacity, like 180 Watts vs. 360 Watts.
3.) You can indeed run two trains on two separate tracks (stretches of track) at the same time. But more importantly if they're Command Control locomotives you can also run them on the same stretch of track at the same time, either coupled to each other, or physically separated. Conventional engines though still work the old fashioned way, usually only one on any stretch of track at a time.
Let us know if you have any more questions.
Mike
