OK, using IDE 1.8.0, which might be superseded now. I have been using ESP8266 Boards/Libraries for a couple years with no problem. Yesterday I installed the ESP32 Board and everything went fine. Then today I went back to the ESP8266 Board and could not find it under Tools>Board or Board Manager. So, I "reinstalled/updated" the ESP8266 Board, closed IDE, rebooted the computer and then the ESP32 Board was not visible. I find that when I go to File>Preferences and put either the ESP32 or ESP8266 URL into Additional Boards Manager URL's and click OK, whichever Board I chose, that is THE ONLY one available, the other is not visible. This seems like a huge error, or deficiency within the IDE framework. Is there a workaround, or must you just go to the File>Preferences depending upon which platform you are intending on using. Thanks in advance for any suggestions.
GaryP explained the solution. You must leave all the Boards Manager URLs in your preferences. If you remove one of the URLs, the boards associated with that URL will disappear the next time you open Boards Manager. If you have multiple URLs, you should separate them with commas.
What I would like to add is that after you fix your File > Preferences > Additional Boards Manager URLs to contain both the URLs, you must do the following before the boards will reappear:
Tools > Board > Boards Manager
Wait for downloads to finish.
Click "Close".
Note that you don't actually need to install anything in Boards Manager, since you already have the boards installed. You just need to run it so that it can recognize the installed boards again.
Thank you both, GaryP and pert. I included both URLs, the boards platform index updated and both boards and their many options appear. A simple solution that I had not seen in any of the ESP32 or ESP8266 tutorials previously.