When a Rainbow Servant hits level 10 they gain access to all nondomain Cleric spells. All spells that do not appear on any other spell list they have are cast as Divine spells. This means that, if they have the skills and have learned 2nd level spells, they qualify for the Mystic Theurge Prestige Class.
But what happens when you actually start to take levels in it? The text says:
When a new mystic theurge level is gained, the character gains new spells per day as if he had also gained a level in any one arcane spellcasting class he belonged to before he added the prestige class and any one divine spellcasting class he belonged to previously.
What it does not say however, here or elsewhere, that these classes have to be different classes. Do you get one level in Wizard/Warmage/whaever for being an Arcane caster, then ANOTHER level for being a Divine caster as well? The text for the Rainbow Servant says the following:
Such spells are cast as divine spells if they don’t appear on the sorcerer/wizard or bard spell lists.
So it comes down to these questions:
- Does being able to cast Divine spells qualify you as a Divine Caster, or is there more to it than that?
- If it is, what happens when you take levels in Mystic Theurge like this? Do you go up one level or two?
- What happens if this means you go over 20th caster level? Do you keep gaining spell slots like a level 20+ character, or does something else happen?