Best way to use multiple SSH private keys on one client

Generating a new SSH key and adding it to the ssh-agent

$ ls -la ~/.ssh
total 12
drwx------  2 andreas andreas 4096 Aug 22 13:16 .
drwxr-xr-x 46 andreas andreas 4096 Sep 22 21:12 ..
-rw-r--r--  1 andreas andreas 1772 Sep 23 11:54 known_hosts

$ ssh-keygen -t rsa -b 4096 -C "your_email@example.com"

IdentityFile ~/.ssh/id_rsa_key1
IdentityFile ~/.ssh/id_rsa_key2

$ git config --global user.name "John Doe"
$ git config --global user.email johndoe@example.com