If you look well at Rashi he is saying the two sins of avoda Zara same as you explain in name of Dubna Maggid. Rashi is NOT saying what you said in his name.

You wrote: "RASHI says that Rebbi Yitzchak's teaching relies on the verse cited by the Gemara shortly afterwards. The verse (Yirmeyahu 2:10) states that the people of the islands of the Kitiyim and the people of the land of Kedar never deserted their gods even though they were worthless. 'One transgression which is equal to two' means that the Avodah Zarah of the Jews at that time was worse than the two idols of the Kitiyim and the Kedarim combined. When those nations went into exile, they brought their idols with them and remained loyal to them. In contrast, when the Jewish people went into exile, they deserted Hash-m and they began to serve Avodah Zarah."

There is no one equal two in such an explanation as evidenced by the struggle to explain this point in above citation. The main explanation of equals two are in last words of Rashi.

Rather he means what you wrote in the name of the Magid of Dubno: "The MAGID of DUBNO explains as follows. When other nations served Avodah Zarah and exchanged their god for another one, it was not so terrible. They merely exchanged nothing for nothing. Since they were justifiably disappointed with their first god and his failure to respond to their prayers, they changed their allegiance to a new god. In contrast, it was a terrible transgression for the Jewish people -- who saw the great power of Hash-m and were the beneficiaries of His great kindness -- to give up Hash-m in order to serve useless and powerless idols. Accordingly, there were two evils in their sin of Avodah Zarah: they left the Almighty Hash-m, and they changed their allegiance to entirely worthless idols in His place."

Daniel Gray, Canada