Could u buy Kurzicks 15k If your guild belongs to the Luxons?

i have Luxon 15k but I want Kurzick 15k as well is that possible?

Yes, just have more Kurzik faction than Luxon faction and you are set to go.

Redeem all luxon faciton. Talk to Kurzick AB guy. Earn Kurzick faction. Kurzick NPCs now talk to you.