What is the relationship between Hattori and Yuujirou? In Season 1 Episode 22, during the KOOGY complaint scene, their full name was mentioned. Hattori Yuujirou and Hattori Akira. Are they siblings or mere coincidence that they share a surname?
It is not stated in neither  Yuujirou's or Akira's Bakuman Wiki pages nor MAL pages   that they are siblings, the conclusion being that they are not. It may be a pure coincidence. I am not very informed regarding this anime though.

Akira Hattori wiki page

Yuujirou Hattori wiki page

also

Akira Hattori MAL page

Yuujirou Hattori MAL page

As you can see their biographies do not say anything about them being blood-related in any way.

Also in Akira Hattori's wikipage it says:


  Personal Status: 
  
  Marital Status Single 
  
  Relatives Unknown   
  
  Education  Unknown


If one of his relatives would have been mentioned (including Yuujiro) they would have been written in the wiki. Thus we conclude that they are not blood-related. 

Moreover this quote:


  The editors in Bakuman are based off of real editors, so the names used just relate to the editors real names. The two aren't related.
  
  http://bakuman.wikia.com/wiki/Talk:Yujiro_Hattori


shows that they are based on real editors and is a common thing in real life for 2 individuals to have the same family name and to not be blood-related.