EDIT: i just found that video:
Antminer S9 - 6 Boards per Controler Modification
https://www.youtube.com/watch?v=D9Pfqcdx6BMlong time ago , Some where on the youtube i watched a video , but i cannot remember if it was S7 or S9 or what ..
but he added/soldered lots of resistors for each extra socket , also on the back of the board
as far as i can remember , that board was kind of like your board , but he soldered the resistors on somewhere like (on your picture) R138 to R140 , and R142 to R144 and ....