I asked a related but different question here: How does the Mystic Theurge's Combined Spells feature work?
In the following scenario, how does a mystic theurge's combine spells ability work? Cleric 3 / wizard 3 / MT 3. They use a second level wizard slot to cast a first level cleric spell. Lets assume that the character has wisdom 18 for +4 and int 20 for +5.
- Which attribute is used in calculating the save DC: the cleric's +4 due to cleric spell, or wizard's +5 due to wizard spell slot?
- What is the DC per the level of spell? Since it's Cleric 1, is it +1 or slot 2 for +2?
The end DC can be 15, 15, 16, or 17 depending on how the abilities work.