This is a tricky question with no answer given in the lore, because any child born to parents from two different tribes is inherently breaking Seeker cultural rules.
By traditional Seeker culture, a woman shouldn't be having children with anyone but the nunh of her tribe - and while they haven't explored it within the story, it sounds like anything else would be viewed as the equivalent of a married person having an affair.
So if your two characters are married (or just together) and keeping their original tribe prefixes, you're already well outside any naming conventions that are available for us to follow. Within the story they're probably having to make it up as they go along just as much as you are.
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Overall it's something I've already put thought into since I was playing with it for characters of my own - or rather I was playing with the naming conventions and the character idea sort of spun out of that, but in any case...
Between the naming conventions page and an old forum thread that gave some additional canon lore information (
here and further down the page, or collated
here), we know there are three ways a male Seeker can become nunh of a tribe, in order of social acceptability:
1. Take over an existing tribe by defeating the current nunh in battle
2. Claim new territory for his tribe and become nunh of the new area (also implying that there isn't necessarily a single "___ tribe" but that they may fragment over an area)
3. Declare independence and make up their own tribe with a new two-letter prefix.
In my case, I made them a fully independent tribe, so Z'aba Tia declared himself Zi'aba Nunh and then I'djalani "joined the tribe" and is now Zi'djalani. It's all within the rules (I think) and they're just a very small tribe that isn't accepting additional members. (The new prefix can be anything but joining the individual names seemed like a cute idea.)
Alternately I possibly could have gone the "claiming new territory" route and have Aba formally claim his apartment in Limsa as Z tribe territory... For all we know, that might be how less-traditional Seekers already get around the rules, though anyone from another tribe would still presumably have to transfer to the nunh's tribe. In any case I prefer the idea of them being a fully separate tribe, particularly when the rules aren't clear enough on how the two sub-tribes might interact.