Both versions are available as zip files. You can find the zip file for 2.80 by clicking "macOS, Linux and other versions" on the download page. The zip file does not require any installation and will allow you to run Blender in parallel to an installed version.

Anyone know of any mirrors with 2.79b or 2.8 zip files to download? I'm having extremely little luck using the blender.org download links. (2.8 official release day: no real surprise there!) Got to 60 or 70 Megs downloaded then died about 4 times so far...

Same Deal. Installed 64-bit Windows version of Blender 2.80 to C:\Program Files\Blender Foundation\Blender\2.80, and it wiped out the 2.79 version installed at C:\Program Files\Blender Foundation\Blender\2.79, including the "2.79" sub-folder of C:\Program Files\Blender Foundation\Blender\. I need to run both versions until all previous work is converted to 2.80 or other platforms, including interactive BGE (Blender Game Engine) apps.

This is no way to welcome existing users to 2.80.

Additionally, I found it curious that, without asking, it adopted (many if not all that it could) of the 2.79 user preferences.

I would actually like to have 3 versions installed. 2.79 as I had it setup, 2.80 as it installed with 2.79 user preferences, and a pristine version of 2.80 that I could customize to my liking. As it is I have an install that has viewport "headers" at the bottom as in 2.79, when I was hoping to switch to the 2.80 convention (still?) of placing such "headers" at the top.

Blend files contain all layout information, including the placement of the headers. In the "File -> Open" dialog there is an option called "Load UI" that will ignore the layout information if unchecked.