Does Piercing Light need two targets?

No, Piercing Light does not require two targets.

According to the card text and the associated RiftJudge FAQ (ID 8481):

• First Target: You must choose one unit at a battlefield to deal 2 damage to.
• Second Target: The second part of the effect says "up to one other unit." Because of the phrase "up to one," this part is optional. You may choose to deal the second instance of damage to zero units if you wish.

Key Restrictions:

• If you do choose a second target, it must be a different unit than the first one, as the card specifies an "other" unit.
• If you choose to use both parts of the effect, you must have two different valid targets. If you only wish to use the first part, you only need one valid target at a battlefield.