My apologies, folks. I did indeed delete some of my posts to shorten this thread before posting my solution. I evidently just made things worse. In any case, I hope the illustrations below help lay out the problem and a remedy.
Let's say I have two Arduinos I want to monitor and edit separately, but also simultaneously. Assume Arduino #1 runs the "Hello World" sketch and is associated by XP with COM12. Arduino #2 runs the "DigitalReadWrite" sketch, and is associated by XP with COM3.
Here's the right and wrong way to do it:
1) Invoke a copy of the IDE, and load the first sketch "HelloWorld," and invoke the monitor. Notice it opens a second window, leaving the original blank for some reason. In any case, it will default to the correct COM port:
2) Now we need to load the "DigitalReadWrite" sketch. However, the mistake I was making was to believe that the new window created when the second sketch is loaded would be independent of the other. It's not. A change in the COM port of one will change the COM port of the other, leading to much confusion and trampling of code. Here's the wrong way:
3) Instead, invoke a completely new IDE and load the second sketch. In fact, I find it helps to physically position one Arduino on the left of my keyboard, and move the corresponding IDE windows to the left of the screen, while the other Arduino is on the right side of my keyboard and so forth. It should look like this:
When the IDEs are set up in this fashion, the downloads and monitoring will be completely separate and independent. It's a really terrific way of troubleshooting handshaking between multiple Arduinos.
My apologies to the forum for not getting this sooner, and for the confusing deletes. I hope someone benefits in the long run.