@xmready, I've used a 12-word seed as an extension in the past. When I was a younger bitcoiner I thought that would double my entropy, but I've since learned that it does not. Take a look at hosseinimr93's post above, he is showing you how to generate an honest-to-goodness 24-word seed with double the entropy of a standard 12-word seed. I also advise against using the same pool of words (i.e. Bip39 word list) for your extension, just to add an extra level of security.
My main motivation behind this post is to split the seed into two pieces in the case of a physical intrusion. A 24 word seed with higher entropy has no benefit over a 12 word seed if the physical backup is stolen. If I break the 24 word weed with 256 bits into two 12 word parts, can I safely store them in two separate locations like I can with the seed extension?