Why should you sign from both addresses? The new address is posted by the conviction that you'll need it to provide a proof of your identity in the future. You should only do the following if you've never posted in this thread above:
-----BEGIN BITCOIN SIGNED MESSAGE-----
I am <user name>. Today is <today's date>. My new signing address is <NEW signing address>.
-----BEGIN SIGNATURE-----
<OLD signing address>
<signature using OLD address>
-----END BITCOIN SIGNED MESSAGE-----