For one thing, the rules for what affects only one vs. what affects both seem arbitrary
For another thing, the text is contradictory on some pretty fundamental things. For example one paragraph states "Dvati twins share one mind" but the very next paragraph states that a pair of Dvati do not share preceptions and must take a full round action to discern their counterpart's mental state. Another pair of contradictory statememts first states "Both twins must simultaneously take the actions required to cast a spell, (although only one must supply material components)" but then states "A lone dvati can cast spells if his twin takes no actions while he casts".
Basically, the article itself can't seem to decide what the link between a pair of dvati is or means.
EDIT:
As an aside, here's some more specific dvati questions
If one dvati in a pair dies and then someone casts regenerate on the other one that's still alive, does it bring the first one back?
What about raise dead with one body dead and the other alive? Does it even do anything? And if it does does it cost a level?
If a dvati pair dies and you have just one of the bodies can you use raise dead on it, or are they considered incomplete unless you have both?
And how do they interact with spawn-creating undead?