Silver Key and Shining Trapezohedron
Discard one of your choice.
Official answer from AEG posted on BGG by Ryan Dancey:

When you're not insane, both cards read: &quot;You must discard this card if you have another card with a number higher than 4 in your hand&quot;.
Since you can't play 2 cards at once in Love Letter, you need to choose which you will be discarding.
If you discard The Silver Key, that's ok. You remain sane.
If you discard The Shining Trapezohedron, that's ok as well. Although since you weren't insane, you now become so (you have a card with an insane effect in your discard).