Add deploy key to checkout from second github repository

From all I could find out, there can be only one deploy key configured under{user}/{repo}/edit#checkout. I need to checkout from a second repository, and a github read-only deploy key would be perfect for that.

Is there a technical limitation that prevents CircleCI from allowing us to add a second deploy key to the same project?

User keys are not acceptable in my case. Machine user keys neither due to my organisation policies.

I managed to make this work by adding an SSH to{user}/{repo}/edit#ssh and some code to wire it up.