One who writes two letters on, whether he did so with his right hand or his left , whether they were the same letter or two different letters, whether he did so using two different types of ink, in any language, he is liable. Rabbi Yose said: One is deemed liable for writing two letters only due to marking, as they would write symbols on adjacent beams of the Tabernacle to know which beam was another beam’s counterpart.
Therefore, one who made a single scratch on two boards, or two scratches on a single board, is liable.
In the Jerusalem Talmud there is an extended discussion of this topic where it is explained that these marks were made to indicate the designated place of each beam in the Tabernacle. Even though all the beams were of equal size, it was not appropriate to align them randomly. It is also explained there that according to Rabbi Yose they would draw diagonal lines on the two beams, which were aligned when the beams were placed together. According to the Rambam, the halakhic difference between writing and marking is a technical one – Rabbi Yose enumerates marking as an independent primary category of labor, and says that one who makes two marks is liable for violating a primary category of labor. Others explain that if the prohibition itself is marking, then one would be liable even when no actual letters are written (Me’iri).
As far as halakha is concerned, basing himself on the Mishna the Rambam rules that one who writes two letters on is liable whether they are two different letters, or the same letter written twice intentionally, or two letters written in any language or script, or written in two different types of ink or paint (Sefer Zemanim, Hilkhot 11:9–10). One who creates two marks, shapes or signs on is liable for writing even if the characters are not letters (Hilkhot 11:17).