EDIT: Changing the initial claims is not exactly what you want, but it should have few side effects, so it's relatively safe to mess with:
1) Open "module_scripts.py"
2) Search for "#set territorial disputes/outstanding border issues"
3) Each line below represents one claim (they are commented)
4) If you want two factions to be mostly peaceful, make sure they do not claim each others' stuff
5) If you want total war between them, make them claim everything the other faction owns
6) You can look up which number is which castle/town in "module_parties.py"
The way this works is that factions will regularly check whether other factions own centers that they believe should belong to them. In the first line, for example, the Vaegirs claim Alburq Castle, which is owned by the Nords. So they will want to attack the Nords to get it "back". If they take it, its slot_center_ex_faction will get set to the Nords (the previous owner), which will make the Nords want to attack the Vaegirs. This pretty much guarantees that there will always be some conflict between Nords and Vaegirs (but the Vaegirs will probably not attack the Nords now, at least for this reason).
If you change it so the Vaegirs claim everything the Nords own, the Vaegirs will continously try to make war until the Nords are completely pushed out of their initial holdings.
Note that each center can only be claimed by one faction (other than the owner). Setting another one overwrites the old value.
There will still be the occasional ceasefire when both combatants are too exhausted to fight, and occasionally wars happen without conflicting claims (e.g. when a caravan is raided). But this should get you mostly the desired effect, and in a way that is unlikely to screw up lots of other stuff.