Raiders can transport up to ten DRUKHARI INFANTRY.
There's no rule about which Detachments they have to be in, or anything like that, and no rule preventing a Kabal unit going in a Cult or Coven aligned Raider.
What you're suggesting, putting two units in one Raider, is absolutely perfectly legal and fine.
This is why some of us are having the conversation as you whether a unit from one Kabal gets the benefit to shooting from a Raider of another Kabal......