Is it obligatory to re-consult the first scholar after obtaining a fatwa permitting something from another scholar, or is the first fatwa sufficient?

It is permissible for you to act upon the fatwa of the one you asked, as long as he is known for his knowledge. You are not obliged to know the fatwa of the most superior scholar, for asking a less superior scholar, if he is known for his knowledge, is permissible according to most usuliyyun (scholars of Islamic jurisprudence principles). The less superior among the Companions and the Salaf used to issue fatwas even in the presence of more superior ones, and no one condemned that. Furthermore, an ordinary person cannot make a judgment due to their limitations.