Trying to setup dual repo SSH config file - github

Question

I've successfully had my MacBook working with my own Github repo for years. Recently, I tried to setup my computer to access both my own Github repo and the Github repo for my work account. I read many articles and threads and learned that this is supposed work with the SSH config file. I've tried many things but just can't get it to work.
In my .ssh directory I have these files:
config
id_ed25519
id_ed25519.pub
id.rsa
id_rsa.pub
The first private/public key is for my work account and the second is for my personal account.
Here's the config file I [naively] created:
Host https://github.com/employer
HostName github.com
IdentitiesOnly yes
PreferredAuthentications publickey
IdentityFile ~/.ssh/id_ed25519
Host RobertWerner-MWTech.github.com
HostName github.com
IdentitiesOnly yes
PreferredAuthentications publickey
IdentityFile ~/.ssh/id_rsa
I've obscured the name of my employer but that http://... address is what my DevOps colleague told me to add. For my personal account, the first part - RobertWerner-MWTech is my Github account name. I don't know if RobertWerner-MWTech.github.com is the correct host name.
Anyhow, it seems that the config file has no actual effect. Even with it in place, I can't access both repos at the same time. The best I can achieve now is to run ssh-agent whenever I want to use a specific repo, like this:
eval "$(ssh-agent -s)"
ssh-add --apple-use-keychain ~/.ssh/id_ed25519
OR
eval "$(ssh-agent -s)"
ssh-add --apple-use-keychain ~/.ssh/id_rsa
While this does allow me to switch back & forth between the two repos, it would be fantastic if I could actually get the config file to work as it's supposed to.
Any guidance on how to resolve this would be much appreciated!
Robert