Nunchuk has something kind of like this, you can set a spend limit per day. So, you'd have your key and they'd use their key and they'd only sign if you spent below your daily limit. You could then put your other key in a nearly impossible to get place (like in a dark cave that you have to offer up innocent blood to find, you then have to cross a dark pool of water in a boat, and then drink magical water that makes you suffer).