From http://www.dailyhalacha.com/m/halacha.aspx?id=284:
One who looks at the text of the Ten Commandments in a Torah scroll
will notice that the Torah assigns a separate paragraph for each
commandment. Even the very short commandments, such as “Lo Tirsah” and
“Lo Tinaf,” comprise an independent paragraph, as do the longer
commandments, such as “Lo Yihiye” and “Zachor.” The “Ta’am Elyon”
system of cantillation notes follows this arrangement, and makes each
commandment no more and no less than a single verse. This means that
the longer commandments are made into a single verse despite their
length, and the short commandments, such as “Lo Tirsah” and “Lo
Tinaf,” are assigned a brief, two-word verse. The exception to this
rule is the first two commandments, which are combined into a single
verse in the “Ta’am Elyon.” The reason for this exception is that,
according to tradition, when God proclaimed the Ten Commandments, He
uttered the first two commandments as a single statement. Therefore,
even in the “Ta’am Elyon,” which reflects the manner in which the
commandments were heard at Sinai, the first two commandments are
combined.
We refer to this system at the “Ta’am Elyon” (literally, “upper
cantillaton”) because generally speaking, the Te’amim that are used to
extend a verse are the notes positioned on top of the word, such as
the Pazer Gadol and Azla Geresh.
This system is used for the public Torah reading, whenever the Ten
Commandments are read. Namely, on Shabuot, on Shabbat Parashat Yitro
and on Shabbat Parashat Vaethanan, the Ten Commandments are read using
the “Ta’am Elyon,” so that they are read the way they were heard by
the Jewish people at Sinai.
Although there is a tradition that a verse cannot be shorter than
three words, we allow separate verses for the two-word commandments in
the public reading, because this is how the Jews heard the
proclamation of the Ten Commandments at Sinai.
The “Ta’am Tahton” system is used when a person reads the Ten
Commandments privately, such as for the weekly “Shenayim Mikra Ve’ehad
Targum” reading or for general individual learning. This system breaks
up the text of the Ten Commandments according to the common,
conventional length of verses. This means that the longer
commandments, such as “Lo Yihiye” and “Zachor,” are each divided in
several verses, and the short commandments – “Lo Tirsah,” “Lo Tinaf,”
“Lo Tignob” and “Lo Ta’ane” – are merged into a single verse.
Also, from http://haemtza.blogspot.com/2008/06/standing-up-for-ten-commandments.html:
The reason for this is the following Halacha: ‘Kol Pesuka D’ Lo Paskiah Moshe, Anan Lo Paskinan’ - we are not permitted to divide Pesukim when reading the Torah - which is a fulfillment of the Mitzvah of learning Torah - differently than the way Moshe did. Sentences read for that purpose must be read whole. The Taam Tachton accomplishes this. Reading the Ten Commandments, however, is more than just a fulfillment of the public learning of Torah. It is a remembrance of receiving the Ten Commandments at Sinai. So we read it with the Taam Elyon to signify that.
The sense, I think, is that while the written Torah has one basic breakup originating at Sinai of the pesukim (hard stops/sof pasuk)*, the ten commandments have an additional breakup in the way in which they were originally presented at Sinai. These two breakups are the taam tachton and taam elyon respectively. (The other trop notes that were a later addition [by Ezra?] follow the convention that is generated when the breaks are far between.)
*I believe Rav Moshe Feinstein, z"l, suggests that the soft stop ethnachta might also be Sinaitic in origin.
