央行行长会: Is the second 行 necessary?
This is an interesting question. My first impression is that there isn't such a term known as "央行长". But after doing a quick search on Google, I realize that there are people who actually use such a term though it isn't that widely accepted.

Let's take a look at the search results from a few similar terms before making a conclusion:


国防部部长 (11,500,000) vs 国防部长 (6,470,000)
协会会长 (8,630,000) vs 协会长 (1,010,000)
肃毒局局长 (31,500) vs 肃毒局长 (21,200)
卫生厅厅长 (6,320,000) vs 卫生厅长 (21,500,000)


As for 央行长:


央行行长 (4,310,000) vs 央行长 (80,600)


Language evolves with time and new terms are constantly being invented while old ones may fall out of use. Some of the new terms may gain wide acceptance while others may not. "央行长" could well be a new term which has not gain much acceptance. Therefore "央行行长" is still the recommended term to use in order to be understood.