Can a character have more than one favored prestige class? What about more than one prestigious spellcaster class? I recall reading "no" initially, but what is the verdict?
The feat itself says you can only have one favored prestige class, but more generally, you can’t take a feat multiple times unless it specifies otherwise, which this one does not. So, it’s a pretty hard “no”.
Note that there's not really a point to having both (or even one) unless going for Prestigious Spellcaster. There's no Alternate FCBs printed for any prestige class so Toughness/Cunning is strictly better.