There was never any question of China joining the Soviet Union, but there was a question as to the extent to which it would be integrated into the Soviet bloc. And the irony here is that when the Chinese wanted greater integration, the Soviets did not--and vice versa! See Odd Arne Wested,
The Cold War: A World History, pp. 144-5:
"One key reason for Moscow’s worry was the Chinese refusal to further integrate into the Soviet bloc, militarily and economically. Up to 1958 it was China that had pressed for such integration, with the Soviets holding back, in part because they feared that China’s enormous population would prove a strain on the Soviet and eastern European economies. But when the Soviet Ministry of Defense in the summer of 1958 had proposed a few relatively routine steps of military coordination, such as Soviet-operated early-warning systems and naval communication transmitters in China, Mao had reacted furiously. “I could not sleep, nor did I have dinner,” he told the surprised Soviet ambassador Pavel Iudin.
"'You never trust the Chinese! You only trust the Russians! [To you] the Russians are first-class [people] whereas the Chinese are among the inferior who are dumb and careless.… Well, if you want joint ownership and operation, how about having them all—let us turn into joint ownership and operation our army, navy, air force, industry, agriculture, culture, education. Can we do this? Or [you] may have all of China’s more than ten thousand kilometers of coastline and let us only maintain a guerilla force. With a few atomic bombs, you think you are in a position to control us.'"
https://books.google.com/books?id=3TBXDgAAQBAJ&pg=PT283
https://books.google.com/books?id=3TBXDgAAQBAJ&pg=PT284
AFAIK, that was the only time Mao mentioned a complete merger of the USSR and the PRC and obviously he did so sarcastically...