"Grafitero" is someone who does graffiti, a graffiti artist. But "bilítero", which means "de dos letras", i.e. two-letter, refers to a "tagger", someone who's graffiti consists of their "tag" or graffiti signature. So although you could say "two-letter graffiti artist", I think this sounds a little strange, and "tag artist", which is a colloquial term for a graffiti artist and specifically for someone whose graffiti is confined to their tag (rather than pictures or "pieces"), would be suitable here.

"Main Entry: tag artist
Part of Speech: n
Definition: a graffiti artist
Etymology: tag 'graffito'
Usage: slang "
http://dictionary.reference.com/browse/tag artist

"TAG
The most basic form of graffiti, a writer's signature with marker or spray paint. It is the writer's logo, his/her stylized personal signature. If a tag is long it is sometimes abbreviated to the first two letters or the first and last letter of the tag. Also may be ended with the suffixes "one", "ski", "rock", "em" and "er". "
http://www.graffiti.org/faq/graffiti.glossary.html

"For those unfamiliar with RD, he’s a NYC bombing legend whose two letter tag quickly became a ubiquitous mark on the city streets during the 80s and 90s. "
http://animalnewyork.com/2013/meta-graffiti-rd-tag-on-banksy...

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Note added at 36 mins (2014-05-30 11:00:30 GMT)
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Sorry: I meant "whose graffiti", not "who's graffiti", in line 2!

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Note added at 56 mins (2014-05-30 11:20:07 GMT)
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If the term is actually "grafito bilítero", the translation is simply two-letter tag.

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Note added at 1 hr (2014-05-30 11:40:17 GMT)
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SORRY!

In my haste I didn't pay proper attention to the context. We're talking ancient monuments here, not modern graffiti.

In this context, I think two-letter inscription would do.

https://www.google.es/search?num=100&safe=active&q="two-lett...

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Note added at 1 hr (2014-05-30 11:57:17 GMT)
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Sorry, but "graphite" is impossible; its only meaning in English is the kind of "grafito" from which pencil leads are made, a soft black substance. It can't refer to a written "grafito".

I think "inscription" is what you would normally use, but I do see the problem about "inscripción" also occurring in the title. So you really need a different term. I think you could use "mark":

two-letter mark

"Mark" is used for letter inscriptions in certain contexts, particularly pottery, precious metals and jewellery, so I think you could well use in here.