As a previous comment stated, Stalin was in power years before Hitler. It is hard to measure how "evil" they were and comparing them is pretty useless. Both of them committed horrendous acts of genocide. Stalin was in power before Hitler so Stalin had a head start, so to speak, on his evil acts. By the 1930's, just as Hitler was gaining significant power, Stalin himself was ordering the mass execution civilians based on ethnic justifications. Stalin's policies lead to massive famine throughout the Soviet Union even before WWII.

Saying they were "not equal due to their beliefs and background" is misguided at best and very harmful at worst. Both Stalin and Hitler were selfish, paranoid men. They were much more worried about their legacy than the well-being of their peoples. Their genocidal acts exemplify that they both were evil men. Genocide is genocide, there is no "nice" way of commiting it. Neither them believed in individual freedom and both of them show that they held true hatred in their hearts.

To answer your initial question, Stalin was a violent tyrant who came before Hitler, who was also a violent tyrant, so Hitler was a second Stalin. Both of them rose to power though violent political suppression. Even before WWII Stalin's atrocities and political suppression were well known. When the academia and politicians noticed that Hitler was doing many of the same things Stalin was, like killing political adversaries, it was fitting to call Hitler a second Stalin.