To behold two creatures of a specified creature type, you can reveal two creature cards of that type from your hand, choose two creatures you control of that type on the battlefield, or reveal one card and choose one creature on the battlefield.

If a cost to cast a spell includes revealing a card, that card remains revealed from the time the spell is announced until the time it leaves the stack.

If an effect refers to a "[subtype] card," it refers only to a card that has that subtype. For example, Glen Elendra's Answer creates a Faerie token and features a Faerie in its art, but it isn't a Faerie card.

If a card in your hand is already revealed (perhaps because it was revealed to pay a cost of a spell that's still on the stack or due to the effect of a card like Telepathy), you may reveal it again to pay the cost of another spell or ability that requires you to reveal a card from your hand.

If a card in a library has in its mana cost, X is 0 for the purpose of determining its mana value.

Notably, Celestial Reunion's behold instruction is limited to "creatures." It doesn't allow you to behold kindred cards or kindred permanents, which are permitted for most other behold instructions.