MArcomage

I recognize that it may be objectionable that a zero-cost common could potentially discard a Rare, but the chances of this happening are low. It would require a lucky Illusion keyword draw or very deliberate use of Doppelganger.

Tweaking the card text. Original text may have given the wrong impression that it discards everything. Is the text clear? (Is "concurrently" too unusual an English word for non-native English speakers?)

The card's text isn't clear to me. I would have guessed that it would discard all cards common to both players' hands. What does it do? Randomly choose one card that both players have on hand and discard all copies of it? Randomly choose one card that both players have on hand and discard one copy each of it?

I think 'concurrently' is confusing only because it's redundant in this context.

You interpreted it correctly. All cards that are present in both hands are discarded. If one player has two Merchants and the other has one Merchant and both players have one Searing fire, all Merchants and Searing fires are discarded from both hands.

I'm just not sure how to phrase it concisely. The sentence "All cards that are present in both hands are discarded" makes sense with the example right after it, but I can see people reading it as "discard all cards from each hand."

I don't want to say "all cards common to both hands" because use of the word "common" would cause rarity confusion. I could make it wordy and say "Any card that is in your opponent's hand as well as yours is discarded." Or is it clear enough without going to such lengths?