How common is infighting between Druchii houses?

[Aristo]

Well, the Druchii are known for their treachery and between noble houses, this is no exception. I ask mainly because the friend who I used to play 40k with is making the jump into WHFB with DE, and I had also been considering them. Whenever I build up an army, I like to write up some fluff for it, and I figured that our two armies would clash as a result of noble rivalries, or something of the sort.
Now, I haven't got the chance to read the new book in its entirety, so I'm not sure what, if anything, it says about Druchii society. Are nobles likely to burst into all-out battle with each other when the going gets tough?

[Calisson]

Consider that cooperation is always possible, based on interest, not based on principle.
Treason is likely to happen if the opportunity presents itself, but it is not a principle either. All the more that all nobles are prudent by nature and would never turn their backs to any ally, so such opportunity does not present itself often.
Note that it is much better to enroll a former competitor and become stronger.

With your friend, it seems natural that as the two houses will be necessarily of same strength, there cannot be any submission from one to the other one.
However, they would not hesitate to ally against a stronger foe if that is necessary (and backstab the weakest survivor as soon as the common threat is beaten).

[Tarbo]

There is no need for treachery to have infighting. Conflict can occur for perfectly legal and acceptable causes, even without a clear transgressor.

For instance, imagine you and your friend as neighbouring fiefdoms with rich mines and forests bearing high-quality wood at your border. Most of the time, treaties and agreements manage who owns what, and reasonable prices are maintained, and so on, and so forth. But once every while, some sort of conflict arises that drives the parties into skirmishing.

Perhaps you have a liege claiming that the resources are legally yours (and thus theirs), or you have a vassal in that particular area laying a hereditary claim. Or local politics could be boiling, turning the populace against their aristocrats, and a person of authority may start a military campaign to create an outward enemy to unite against.

The actors in these hostilities may be rivals, seeing each other as competitors in the greater world, or they may usually get along quite well, even having solid diplomatic relations, if it weren't for those pesky lieges/vassals insisting that their twice removed family's heritage and honour be defended from some slight or another.

In summary, yes, I expect there to be infighting to a certain degree. Any point of conflict, from trade levies to resource competition to family feuds to outside forces, can be a one-day talking point or an outright casus belli. Exceptionally belligerent nobles, however, would be unlikely to last long, as their peers and superiors may band together to fight the common threat--nobody wants to live next to Genghis Khan, after all.

[Daeron]

Another reason to attack one another is... weakness. Druchii do not tolerate weakness. If one noble's house becomes weak for some reason, then it may find itself targetted by another house. The rationale is that you should be powerful enough to keep what you have.
Even if the moment of weakness would be temporary, another Druchii might take the opportunity to strike the house down and claim their possessions.

And of course.. rivalries can exist. Take the Cult of Khaine and the Cult of Slaneesh (the pleasure cult) for example. Houses can be bitter rivals too.
That said, a rivalry between Houses could be dropped in an instant if the opportunity is right. Rivalries between the cults on the other hand would not be dropped so easily.

As an example.. If my memory serves me well, they would raid Brettonia as much as ally with it. How? Well, they might agree with one Lord to raid only the land of another Brettonian Lord. What Lord would deny such an opportunity to climb the echelons of nobility at the cost of a rival, while guaranteeing safety for his own people? And while the Lord distracts his rivals' army on land, the Dark Elves raid the rival's coast with ease. If they find themselves insufficiently rewarded for this partnership, then they just raid their ally's coast as well as he's out on the land distracting his rival.
Next year they strike the same deal with the (desperate?) rival... For perhaps he would succeed where the other one faltered. And why not take an opportunity to strike at your enemy so, whilst safeguarding your own people?
What if this rival Lord would have grown smarter? Well then, now he has two enemies to worry about, because the Druchii will not tolerate such defiance for long.
They will use any tactic that befits the need.

In short: Druchii kill and murder anything and everything... except if they fear to suffer from the consequences. That is how the discipline is kept in the ranks.
Druchii can ally with (almost) anything and everything because they are extraordinarily gifted in persuading an enemy to a common cause. But no ally is truly safe with a Druchii at his or her side.

[Saintofm]

It is common enough for a social etiquette to require commoners to be within 4 sword lengths, body guard and retainers 2 sword lengths, trusted retainers 1 sword length apart (a sword length is about a yard or meter, about the length of one's outstreched arm to the tip of their blade). Anything closer are for playthings and rivals.

While open war would be a bad thing (kills to many good troops, and might make Malekith think you're planning on attacking him), it does happen from time to time. They just need to find some form of justifacation and wala, they can try and wipe eachother out.