Basically every week it's almost a guarantee that you will get a key off your first rare for the week. If you get a chest instead, same deal, you usually get a key. But, you can only get one off of a rare or a chest. So if you kill a rare and get a key, if you loot a chest somewhere on the isle, you won't get a key, and vice versa. Now a second key is possible from an arcane trove, but it's a low chance. Between 2-4% apparently. I've gotten one from an arcane trove before. Being that you can get two arcane troves in stage 4, you have a chance of getting 3 keys at the start of every Tuesday, though it would be insanely rare. lol Basically just continue doing the dailies every day and hope you get another key for the week.