So you are telling me that if someone asks the mystic theurge if they are a divine caster they should say no because they are a divine caster and an arcane caster?
I am afraid that I dont see that as being true. They are a divine caster, they are an arcane caster.
if you have a portal that only divine casters may enter then can the theurge enter?
if you have a portal that lets everyone through but arcane casters may not pass may the theurge enter?
The theurge is both divine caster and arcane caster.
If someone is a divine caster and has levels in anything else (assuming the anything else does not remove the divine casting abilities) then they are still a divine caster.
Sure, they may not be as strong of a divine caster as other divine casters, but having other talents does not make them 'not' a divine caster.
If I have a special dart gun that only shoots ballons that are yellow and there is a balloon that is both yellow and red it will pop it. Why? All that the gun asks is 'is the balloon yellow'. The red part of the balloon is still popped.
If the item here had said 'divine only spells' then there wouldnt be an issue, but it doesnt, it says 'all of his spells', his refers to needing to be a divine caster.
If you wish to say that divine casters are casters who can only cast divine spells then that is your call of course, but it seems like a very strange limiting condition to me. Does that mean that if they ever pick up some sort of use activated item which says something like, 'this item allows you to cast spell X Y number of times per day' that they would no longer qualify as a divine caster if the spell in question is arcane?