Why did stalin declare war on japan? what are your thoughts on the fact that he did so two days after the united states dropped the bomb on hiroshima?

At the Yalta Conference in February, 1945, Stalin had agreed to enter the war against Japan three months after Germany was defeated.  Victory in Europe was achieved on May 8, 1945.  The Soviet Union declared war on Japan on August 8, 1945, and invaded Manchuria with over a million troops to take on the Japanese army there.

As to the dropping of the second atomic bomb, even the dropping of the first could be challenged when factoring in the USSR.  An option to dropping atomic bombs was to enlist Soviet troops in a joint invasion of Japan.  But the USA wanted to avoid postwar Soviet presence in Japan, and the atomic bombs were seen as a way of ending the war quickly.  As to the use of a second bomb at Nagasaki after the first was dropped on Hiroshima, it was because of the Allies' requirement that Japan submit to an unconditional surrender.  They did not do so in the immediate aftermath of the Hiroshima bombing, so the second bomb was used.  You can consider for yourself whether some other resolution besides "unconditional surrender" was a viable option.