I am curious as to why the nos dry system has 2 nitrous solenoids, 1 before and 1 after the vacum line regulator. Is it to act as a check valve for the vac lines?

 

The second solenoid is a saftey solenoid that would be wired through a FPSS normally. So that it doesn't open and allow nitrous to go into the engine if there is not enough fuel pressure. But if I rember right the 05176 kits didn't come with a FPSS since they had a fuel pump. They may have started adding them at some point though.