It actually makes Deathwing somewhat useable, and is the wonderful bit about the way detachments work. Arguably, Deathwing needed something like that mechanic (i.e, only 1d6 scatter anyway) without the need for the Inquisitors.
In regards to getting them to land on target, he only rolls 1d6 less no matter how many Servo Skulls he is within range of. The rule says
"A friendly unit arriving by Deep Strike rolls one D6 less for scatter if it aims to arrive within 12" of a Servo-skull. Likewise, friendly blast templates placed within 12" of a Servo-skull roll one D6 less for scatter."
If you land within 12" of a Servo Skull, you roll 1d6 less.
Think of it this way.
You have the two Skulls 18" away. Around Skull A, you have a 12" Circle, and around Skull B, you have a 12" circle. If you in neither A or B's area of effect, you are not within 12" of a Skull, so there is no effect.
If you land within A, but not B, or B but not A, you are within range of a skull, so roll 1d6 less. If you land within A and B overlap, you are still within range of a skull.
There's no mention of what happens when you're within range of multiple, so you revert to the permissive ruleset baseline which is that you do exactly as the base rule says;
Basic Versus Advanced
Basic rules apply to all the models in the game, unless stated otherwise. They include the rules for movement, shooting and close combat as well as the rules for morale. These are all the rules you’ll need for infantry models.
Advanced rules apply to specific types of models, whether because they have a special kind of weapon (such as a boltgun), unusual skills (such as the ability to regenerate), because they are different to their fellows (such as a unit leader or a heroic character), or because they are not normal infantry models (a bike, a swarm or even a tank). The advanced rules that apply to a unit are indicated in its Army List Entry. Army List Entries can be found in a number of Games Workshop publications, such as a Warhammer 40,000 codex.
Where advanced rules apply to a specific model, they always override any contradicting basic rules. For example, the basic rules state that a model must take a Morale check under certain situations. If, however, that model has a special rule that makes it immune to Morale checks, then it does not take such checks – the advanced rule takes precedence. On rare occasions, a conflict will arise between a rule in this rulebook, and one printed in a codex. Where this occurs, the rule printed in the codex or Army List Entry always takes precedence.
So, in this point, as you can see, there's inclusion of multiple, just "a" which implies landing within a 12" area of a skull. You are within a 12" area of a skull, it just so happens that there are 2 there. But you are still within a 12" area of a skull, which is all that it checks for.
If you think of the rule as an on/off switch, or a logic gate in an electrical circuit.
The rule requires 12" within a servo skull; if yes, -1d6 scatter.
There is no rule for are you within 12" of two servo skulls; if yes, -2d6 scatter.
And if that's how he wants to play, pick up your own deep strike counters or similar; Drop Pods or similar. Not sure if there are other 1st turn deep strikes available to Chaos (I assume you're chaos?)
