Bonus Types: Usually, a bonus has a type that indicates how the spell grants the bonus. The important aspect of bonus types is that two bonuses of the same type don't generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works (see Combining Magical Effects). The same principle applies to penalties—a character taking two or more penalties of the same type applies only the worst one, although most penalties have no type and thus always stack. Bonuses without a type always stack, unless they are from the same source.
I see no reason these two feats would not stack. Divine Protection's special clause doesn't even kick in since steadfast personality doesn't apply to all save throws, only will.
Of course your wisdom modifier still applies too so this could be a way to have a low wisdom high will character (stubborn as a mule and only half as wise!)
I would suggest the best argument against it working is would be the "Same effect different strengths" clause of stacking effects (same effect in that it is charisma to a save throw, different strengths in that it's only one save throw type instead of all save throws).
But that is a really big stretch since one actually has a bonus type and the other is untyped.