Here's a thought which might help with the generalization point (see the end of my last post).
Suppose the case is as before except your credence in (p iff q) is 0.9 rather than 1. Then you aren't sure that your evidence concerning p is to be treated as evidence concerning q and vice versa, but you should think your evidence concerning p serves decently well as evidence concerning q and vice versa. Your credence in these two propositions therefore should not be able to get *too* far apart, else it will look like one of them (at least) is being influenced by something other than your evidence. Weakening your credence in (p iff q) correspondingly increases how far apart your credences in p and q can get before it starts to look like some untoward influence is at work.
Another point (thanks to Daniel for this): the reasoning I'm using here and in the original case involves an assumption that variation in credences without variation in evidence suggests that one's credences are sensitive to something other than evidence. One way of denying this would be to claim that evidence (sometimes) makes a range of different credences in p admissible but does not single out one in particular as correct.
It is not surprising that those who hold this permissive type of view find it harder than others to account for the strangeness of having two different credences in (what you know to be) materially equivalent propositions. They'll presumably even find it harder to account for the strangeness of having two difference credences in (what you know to be) one and the same proposition. Maybe permissive people should doubt whether there is anything epistemically wrong with incoherent credences. That wouldn't, I think, undermine my explanation of what sort of account people who think there is something wrong with it should give of that wrongness.