So, we have the secretary of the Communist Party (therefore known as the PCUS). Gorbachev was the secretary since 1985. Yeltsin was the president of Russia, not of the whole USSR. Yeltsin didn't agree with Gorbachev's Glasnost and Peretroiska. Yeltsin and other members of the Nomenklatura (the elite of the PCUS) decided to take Gorbachev away from power and divide the USSR. The "coup de force" failed, but seeing the imminent end of the USSR and practically not having support from the PCUS, Gorbachev decided to end the USSR.

Technically USSR was the Union of 15 Soviet Republic and the right to secede from this Union was declared in Soviet constitution. By 1990 (when 1st President of the USSR was elected) several republics already seceded (Baltic States + Georgia and Azerbaijan).

First (and last) President election took place in USSR during March 1990, while there was no President of Russia for more than a year after that, there was attempt on behalf of Gorbachev to create new Union treaty (and most of the remaining republics were agreed to sigh it), but Coup d'etat in August 1991 stopped this process and spurred the collapse of the USSR.

USSR was itself vertically integrated state, and Yetsin position (as the Chairman of the Presidium of the Supreme Soviet of Russia) technically was below Gorbachev's in chain of command, and I say that Yeltsin himself benefited most in the outcome of the failed Coup (he became the first President of Russia two months before the Coup), and later along with leaders of Belarus and Ukraine Yeltsin disintegrated Gorbachev's plan to sign new Union treaty. And basically that was it, seizing direct control over army and state security as the result of failed Coup in August 1991 and refusal to sign new Union treaty left Gorbachev with nothing to rule over.

Yeltsin was president of the Russian Republic. Gorbachev was Secretary of the Communist Party. Two totally distinct positions.