I am not aware of any way to manually designate primary path. For SAS shelves primary path was always selected on whole shelf basis, but different shelves in a stack normally are balanced between two adapters. I think I have seen different output, but it could have been in maintenance mode. I just briefly checked and all systems I have access to have whole shelf assigned to one primary path. System with 6 shelves in stack has them balanced between two adapters (3+3).
That’s different from FC shelves where primary path was selected per disk.
Re: cDOT strange disk assignment on 3rd disk shelf of a stack