riddle - Giving Gifts to Twins

If you give the first twin a gift, he will attach two letters on one side and give it back to you.

If you give the second twin a gift, he will ignore it and show you his ID.

Despite being twins, these two are literally opposites.

Who are the twins?

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Hint #1:

One of the letters attached is not an English letter.

Hint #2:

ID means "identity," not the literal letters "ID" as some answers have tried.

Hint #3:

This is not wordplay, although the concept underlying this question has applications in linguistics (or so I'm told).

Hint #4:

When you give someone a gift, they will always return a gift. Also, there is no distinction between gifts and giftees.