Hay, I am late to the party but, here is my limited understanding of the subject:
Feruchemy stores your attributes, Allomancy is not yours, you kinda borrow it from preservation. So if you are a twinborn A-pewter and F-gold and store health let say 9 out of your 10 health points, then you burn pewter, and get like 8 more, great, now you feel better, but I don't think that extra health is store able.
Well that's how I see it anyway.