Why are the Manchurians who established the Jin dynasty referred to as "Jurchens" while Manchurians who established the Qing dynasty are referred to as "Manchurians"? Why two separate names?

To put it simply, they were the same people.

According to Mark C. Elliot's The Manchu Way: The Eight Banners and Ethnic Identity in Late Imperial China, 47-48:

In the search for Manchu origins, the most important point to note is that in the beginning, there was no Manchu nation, only many Jurchen tribes. The name "Manchu" and the creation of a unified state came about a virtually the same time, in the 1630s, prior to which time it is correct only to speak of Jurchen (Nvzhen/Jusen) peoples, Jurchen tribes, Jurchen leaders, and Jurchen institutions....

....The core of what would become the "Manchu" people - that is, the people at the center of the banner system - were originally either Jianzhou or Haixi Jurchens, along with two of the Yeren tribes; the rest of the Yeren were never integrated, socially, politically, or economically, into what later became Manchu structures.

It's worth noting that prior to the establishment of the Qing, Nurhaci was the Khan of the Later Jin (後金), which he claimed to be the successor state to the Jurchen Jin dynasty.

On page 63, Elliot talks about the creation of the Manchu identity under Hong Taiji:

These efforts at consolidation came to a climax in the mid-1630s, a major watershed in the development of the Jurchen state. First, the name "Manchu" was officially adopted in 1635 as the name for all Jurchen people.

The use of the term "Manchu" served to forge a new pan-Jurchen identity. This new identity was used to unite the divided Jurchen tribes into a single entity to combat the Ming.