How to import a secret subkey?

[Adam Funk]

I recently created a new subkey for a keypair that I use on two 
machines, but I cannot get the subkey onto the second machine.  I have 
tried gpg --export, --export-secret and --export-secret-subkey on the 
first computer but gpg --import refuses to add the subkey on the second 
one.

How can I do this?

Thanks,
Adam