So in a few of the supplements and adventures they have npc's with 2 class grafts in their write ups, but I can't find any rules for if I wanna make my own dual class npcs? Can anyone tell me where I can find said rules?
I believe that you would apply both class grafts, and then split the NPC's number of special abilities between the two lists. You could add all of both sets of special abilities, but you'll likely end up with something a little too complicated to run, or too powerful for CR. It's a judgement call.
Yes, it does involve splitting your CR into a separate 'effective CR' for each class graft. You also use any special rules from both grafts (like solarian attunement) and select gear as though the character had all the proficiencies that would come from the two classes.