Okay, so my MC and his twin need to have a place where they will both have equal access to money on their 18th birthday. I was thinking a bank account. Is it possible that their dad could've put both their names on a bank account but not specify how much money one person can get. (The MC is supposed to take all of it) If this situation isn't possible, is there something else I could have that would end up with the same effect of the MC taking all the money and leaving his twin nothing?
With a joint bank account either party can wipe it at any time... probably not how the father would have set it up if it was a legacy/inheritance thing. A trust like @GingerCoffee said would be better for that. Not too sure about the details though.