Zoidberg: Good question, multiple logs support even within the same Windows Account?
Well, what happened when I tested it last night, was it logs on with the current logged user of the requested client. In my case, I was logged into Galaxy with my GOG account and all my personal client accounts, and my son is logged into Epic Games Store. I launched Fortnite from GOG Galaxy, and it opens Fortnite through EGS client, like usual, but on my son's account as he is the one currently logged on EGS. So it will open a game in the requested client with the current login for that client.
And yes, my son uses the same windows login as I do because we're a little old school. A single offline login. No Microsoft account attached.