There is a class or two that casts both arcane and divine spells, but they are not that great (mounteback or something in the dragon compendium).
One trick I like, but maybe too cumbersome for your build is Eldrich master. This PrC is in dragon magazine #280 and requires you have the ability to cast arcane spells to get into the class. It does not advance spell casting, but at level 4 it lets you add another classes spell list to your own list. These spells are all cast as arcane spells. The wording on the ability also does not say you get to add the additional list to just one spell casting class. I would argue if you cleric had a lists of spells that he cast as arcane spells without using feats to do conversions on them, then the cleric is both an arcane and divine class.
If this is the case you could take the Arcane schooling feat from Faerun that lets you cast a limited number of arcane spells a day to qualify. Throw in santium spell or versatile spell casting for entry into mystic theurge
Cleric 3/Favored Soul 2/ Eldrich Master 4/ Mystic Theurge or Ultimate magus 11
or
Cleric 7/ Eldrich Master 4/Divine Champanion 1/ Mystic Theurge or Ultimate Magus 8
I cannot think of a way to get 9's in both classes, but you should be able to get 9's in one of the classes by double progressing it with mystic theurge since the class is both an arcane and divine class.