Tom, could you give me an example of this case?
Because as I understand it, you need to have different tribes in the same tribal box, and I don't know how it could happen, as by the Hostile reactions each tribe have to be placed in it's own tribal box (so matching with its name). So if you will have always the same tribe map units in the tribal boxes...how it works?
Internecine War is triggered when you place a third Tribal Unit in the box for that particular tribe and there isn't a leader present. For example, when you place a third Silures unit in the Silures box, it triggers Internecine War.
At that point, you want to check the row for that tribe on the Internecine War Table, and determine if there are any units belonging to the Primary Target tribe on the map; if not, you check the Secondary Target. To continue the example, the Silures would check first if there are any Demetae Units (who would be in the Demetae Box), and resolve the warfare against the top-most Unit in that box. If there are no Demetae Units, then you'd check if there are any Ordovices (Secondary Target for the Silures).
Let me know if that clears it up.