I grabbed Thistle's hand and put it around the key, and tried to turn it. No good. Nothing happened. Then I grabbed Penny's hand, did the same thing, and it finally turned.
Ok so that was how it worked. I grabbed Thistle's hand and made her turn the key in the lock of the other chest. It also turned. Both chests opened and revealed the cure all medicine.
I snatched them both and shoved them into Midas. I need to manifest him more. It was just such a drain on my mental resources to do that and tell him what to do all the time. It was so much easier if I just did everything.