The Tanach Study Center has a full discussion.
Key points extracted: (note the use of the word "may" at critical points).
The Shtei HaLechem is the special korban of Shavuot.
It is the only korban 'mincha' offered by the tzibur that is baked as
'chametz.' (All other flour offerings must be baked as 'matzah.')
It is the only time during the entire year when the tzibur brings a
korban shlamim i.e. the two k'vasim that are offered with the Shtei
Matzah symbolizes the initial stage of a process, whereas the fully
risen 'chametz' symbolizes its completion. Thus, the mitzvah to bake
the Shtei HaLechem as 'chametz' may indicate that Matan Torah should
be understood as the culmination of the redemption process that began
with Yetziat Mitzrayim. Just as the Shtei HaLechem marks the
culmination of the wheat harvest, the staple of our physical
existence, the historical process that began with the Exodus
culminates with Matan Torah, the essence of our spiritual existence.
The first instance where we find a korban shlamim is at the end of
Parshat Mishpatim Shmot 24:4-8 when the Torah describes the special
covenantal ceremony which takes place at Ma'amad Har Sinai. At this
ceremony, Bnei Yisrael proclaim "Na'aseh V'nishma" while entering into
a covenant to become God's special nation by accepting the laws of
Matan Torah. That ceremony included the offering of special korbanot:
olat and shlamim (see Shmot 24:5). The blood from these korbanot,
sprinkled both on the mizbayach and on the people, symbolized Bnei
Yisrael's entry into the covenant (24:6-8). Thus we find that the
very first korban shlamim is offered as a symbol of Bnei Yisrael's
acceptance of Matan Torah. A shlamim reflects a joint feast shared by
covenental partners. Therefore, the korban shlamim that is presented
together with the Shtei HaLechem on Shavuot may serve as a symbolic
reminder of Matan Torah.