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?

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 , although the concept underlying this question has applications in linguistics (or so I'm told).

Hint #4:

• The only words which consist of "gift" with two letters on one side are GIFTEE and GIFTED. Not sure if this is relevant. Dec 8 '16 at 2:01
• This feels like network communication protocols stuff. Dec 8 '16 at 2:20
• I'm not sure if I made this too hard :) Dec 8 '16 at 2:34
• @randal'thor REGIFT could also work (you can regift something you received) Dec 8 '16 at 13:08
• This question has been inactive for several months now. Would you care to give another hint?
– John
Mar 9 '17 at 20:36

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

Resign (to quit) - And add two letters for resign(ed) - Having quit.

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

Instead of a signature, or actually a second signature, they might show you their ID instead.

Despite being twins, these two are literally opposites.

I've got a feeling that the second twin is

PRESIDENT

because if you give him a PRESENT and he ignores it you get PRESIDENT - PRESENT = ID

The best guess I've got for the first twin is (given the new hint)

REPRESENT
RÉPRESENT

because you add two letters to PRESENT to get it, and in French répresent means reply, which is sort of "give it back".

In a way, these are opposites since

A president doesn't need to be represented, because he's already the president? (Streeeeeeeeeeeeetch)

• How are they twins?
– Sid
Dec 12 '16 at 15:10
• You might want to brush up on your french, as this is not an actual french word. "représenter" is a verb that resembles the word you wrote, but it does not have the meaning you attribute to it, meaning more or less the same as "represent" in english Feb 13 '17 at 14:36

Seems unlikely, but could they be:

The words 'PRESENT' as in a gift (give a present) and to show (present arms)?

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

Adding "RE" to "Present" can mean "RE-PRESENT" which is to show something once again

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

Thinking of signs in shops that say 'Please present identification before buying etc etc'

Despite being twins, these two are literally opposites

Not so sure how they are opposites but in the former you are giving of your own accord, while in the latter you have to be asked for it?

• Interesting interpretation, but not the one I was going for... Dec 8 '16 at 2:35

Given that the OP says that this question is undergirded by a concept in linguistics, it may be that he is referring to the linguistic concept of Siamese twins, collocations comprising two open class words, often connected with a conjunction. For instance,

• give and take
• right and wrong
• cat and mouse

Many Siamese twins comprise antonyms (e.g. give and take and right and wrong, above). The fact the OP says that "despite being twins, these are literally opposites" suggests that we should be looking for an antonymic pair of Siamese twins.

The non-English letter of Hint #1 may be referring to an accented vowel, e.g. é.

Still working on the explanation part but is it

ENTER and ESC key

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

ENTER key just returns the control after moving cursor to new line which in programming is mostly referred as '\n' hence the two letters

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

Some special function of old keyboard layouts ?

Despite being twins, these two are literally opposites

Self explanatory

• Hover over suggestion #1, suggestion #2 (relevant Wikipedia reference),  … (Cont’d) Dec 9 '16 at 18:54
• You could also see a newline as \r\n which is two letters / characters... Not the answer I was going for, though. Dec 9 '16 at 18:54
• @Challenger5: Ha! Ninja'ed! Dec 9 '16 at 18:56
• (Cont’d) …  and suggestion #3. Dec 9 '16 at 18:57

The Twins are

too and two

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

Despite being twins, these two are literally opposites.

two represent small amount and too may represents too many.

I am stuck after here..

• I suspect that “tow” is a typo, but, since I don’t really see where you’re going with this, I didn’t fix it. Dec 9 '16 at 18:58

present and present

Why

if you PRESENT someone a PRESENT, you become the PRESENTER
if you give a PRESENT to the PRESIDENT, he shows you he already has PRESENT but it has ID too.

The Twins Are:

Re & Er

Reasoning:

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

If you give the second twin a gift, he will ignore it- and maybe even grunt a bit (er, er)- and show his id.

Despite being twins, these twins are literally opposites. Literally, they are a prefix and a suffix.

Who are The Twins?

Re & Er are the twins. Re is the first twin and Er is the second one. They are located at the opposite ends of the word 'regifter' (suffix & prefix). The letters of each twins name is in reverse of the letters in the other twins name. re-er, er-re). Re also appears to be enough in touch with his surroundings to be able to make reference to them (re = regarding) where as the 2nd twin Er is either indifferent or has no clue to the going ons about him (er, er, uh, uh,).