A few things:
Twinborn means a misting that is also a ferring, aka 1 allomantic power, 1 feruchemical.
Compounding works by rewriting the 'key' of the metal. I won't explain this the best, but I will try. So allomantic abilities can only be used when you have the right 'key' (or metal). If you store feruchemical stores in the metal (they have to be keyed with your identity), it rewrites the 'key' to that of feruchemy.
TL;DR bronze gives you seeker powers, but if you store it, it will give you amplified wakefulness, rewriting the power with that of the feruchemical power, giving the allomantic net positive with feruchemical abilities