The sefer Imrei Chein explains these opinions. They are arguing as to who is the nishal - to whom are we asking the question. Rashi holds we are asking the kohen gadol and that is why the asker faces him and that explains why he must hear the question. Rambam disagrees and holds that the question is to the Shechina. The kohen is only a conduit to the Ribono Shel Olam. This is why they both are facing the aron, the place where the Shechina resides and this explains why the kohen does not have to hear the question.
Look in Ibn Ezra in Parshas Pinchos where the Torah informs us of the asking of the Urim V'Tumim and he says two explanations in the words of the passuk 'al piv yetzu'. One pshat is by the mouth of Elozar HaKohen through the Urim V'Tumim and his second explanation is by the words of the Urim V'Tumim. The two explanations are based on what we explained before.
According to this, perhaps we can explain a Gemora in Temurah. The Gemora relates that in the days when Klal Yisroel were mourning the loss of Moshe Rabbeinu, three thousand halochos were forgotten. They went to Yehoshua and Elozar (and others) and said "Go ask from Hashem these halachos." There were two responses given. One was that a prophet does not have permission to create new laws and a second answer to them was that Torah cannot be found in the heaven. What is the significance of these two responses?
The Brisker Rov asks on how would they have asked Hashem anyway? Moshe was the only one who had a direct channel to Hashem. They couldn't speak to Hashem whenever they wanted? He answers that the Gemora means that they would ask by utilyzing the Urim V'Tumim.
We now can understand the two responses of the Gemora (bderech drush ktzas), based on the two explanations of the Urim V'Tumim. If the question was asked to the Ribono Shel Olam and the kohen is only a conduit, the response that was given to Klal Yisroel at that time was Torah cannot be found in the heavens. A halachic shaila cannot be answered using the Urim V'Tumim. If the question was asked to the kohen and he would answer using the Urim V'Tumim, his response was that a prophet cannot create new halachos and therefore he doesn't have the power to return those halachos that were forgotten.