How to share one pgp-key on multiple machines?

In a terminal, run the following:

gpg --export-secret-key -a > secretkey.asc

And on the other system, import the secret key with:

gpg --import secretkey.asc

Alternatively, if you've got ssh access to the other system you should be able to combine these two actions into a single command:

gpg --export-secret-key -a | ssh othermachine gpg --import -

Once the keyfiles have served their purpose, securely delete them:

shred secretkey.asc && rm secretkey.asc

or

shred --remove secretkey.asc

Make sure to shred and remove the key instead of using normal deletion. Additionally, instead of moving the keyfile with mv, copy it, then shred and remove the original. These methods will prevent an attacker from recovering the key through low-level bit inspection.