This is a good article for how to get multiple accounts, with differing ssh keys, to work with github: https://medium.com/@xiaolishen/use-multiple-ssh-keys-for-different-github-accounts-on-the-same-computer-7d7103ca8693
I started with this, but this scheme means I can't just cut-paste the repo link from browser to command line; I'd have to rewrite the host every time I want to use the second key. Example:
# This will always pick up my main ssh key
git clone [email protected]:Hefeweizen/junk.git