This is the 720th most frequent Japanese word.
両国
"Ryogoku" - A district in Tokyo, Japan.
両国 refers to the two countries involved in cooperation.
両国は互いに協力し合っています。
The two countries cooperate with each other.
両国 is used to specify the subject of study, namely, the two countries' relationship.
彼は両国の関係を研究しています。
He is studying the relationship between the two countries.
両国 is used here as a proper noun, referring to the Ryogoku area in Tokyo.