第一个that是形容词,做assembly的定语
which指代assembly,做establishes的主语.
第二个that是宾语从句的连接词.这里establishes==shows / proves.
sth establises that + 句子 仔细看吧,结构是 that assembly -- -- -which - - - - -
that的使用在於“强调”assembly.
that assembly -- -- -that / which - - - -
-== the very assembly -- - -that -- - -
She is that girl who lives next door.
== She is the very girl that lives next door.