I'm in search of information concerning the relations between Navigators and Astropaths.

In the midst of character creation for my newly established playgroup, two players quickly expressed interest in being twins. I decided to allow this provided they agreed on a homeworld and created the appropriate backstory. However, when choosing career paths, one became a Navigator and the other an Astropath Trancendent (neither of which were duplicates paths of the other players).

I've been approached by one of the two questioning the validity of their class choices while being twins. After some searching, I was unable to find anything about the relations between the two paths. I'd really appreciate the help of the community in finding (or improvising) some information.

While I'm no expert on navigators by any stretch, I don't think it's likely for them to be twins. navigators are incredibly selective about their breeding to make sure that they remain pure and the navigator gene breeds true. Now, it certainly wouldn't be IMPOSSIBLE, perhaps the gene takes in one, but not the other, but for one twin to be born without the navigator gene and then become a psyker is at least grounds for immediate excommunication if not outright execution. Someone would have needed to smuggle the psyker twin out of the family, and depending on how quickly the Twins' parents were able to deduce that one of them was a nascent psyker, it's a very real possibility that the navigator twin may never have known he (or she) had a sibling until encountering them on the rogue trader's vessel.

As I understand the rules, they can't. Navigators are a recessive gene, and so both parents must be active Navigators. In that case, both SHOULD have the Navigator gene, and that hedges out the psyker gene. So, what might one do? Here are two thoughts I had. First, you tell the stuff I just said that it's your game, and make it work. The galaxy is a vast place, filled with bizarre phenomena, and if they are not identical twins, they don't have the same DNA, so one of them might have developed the recessive gene, and the other only a carrier for it. If they are IDENTICAL identical twins, then maybe the one has a genetic abnormality, which developed during pregnancy. Option two, if your mother is a Navigator, and slept with your father, who also is, you might be a Navigator, but if she also slept with another man the same day, say maybe one of the gunnery crew, and a second egg also got fertilized, you could both be "twins", born on the same day, but have wildly different traits.

As antijoke said it seems fairly unlikely to have happened. Navigators frequently (if you believe the rarity of the Navigator gene) have non Navigator children. These often grow up to be house operatives or fill many of the non-navigator roles that the house requires. However a Navigator birthing a psychic would be practically unheard of.

I might allow it if the Astropath twin took something in the origin path that could explain how they were exposed to malign influences and some how developed psychic powers independently of their Navigator genetics.

[Edit] They would have to be Fraternal Twins though, not identical twins unless the Astropath Twin was a Navigator but some pre-pubescent accident damaged their warp eye before it could open and then they engaged in some serious heretical research/experimentation to tap into the power they thought they had been cruelly denied... actually I'd let that backstory fly. They'd have to be cast out of their Navigator house and never spoken to again (officially).

Thanks everyone. I'll send them a link to this thread. From what I understand, they were leaning more towards something like what WeedyGrot mentioned in their edit.