Is this possible?  It seemed to me that you have to set the project as HTML or Downloadable...and I see you can upload multiple files (I'm assuming if you use butler and specify a different channel that also works...), but I don't see how to have both downloadable and webGL.

Yes, it can be done easily. First, set the kind of project to HTML,

When you upload the zip with the index.html file, tick the "This file will be played in the browser" checkmark

Change the embed options to your taste

Then upload any other additional files and tick the corresponding OS checkmark.

Save your project page and it's done. The download buttons will appear at the bottom of the page, while the game will be embedded at the top.

You can see an example of how this looks in one of my games: https://remruts.itch.io/wssd
You can also customize the theme of the game page to suit your needs.

Thanks very much!  I see, that makes sense, I wasn't sure it would distinguish if setting the project type to HTML.  Will give it a go :)