Well, particular types of spells aren't tied to specific power sources. Bards for example have been able to cast arcane cure spells since 3rd edition, for example. So if all you are looking for is a witch-type character who can truthfully say her spells come from arcane and divine sources, then you can implement that without any changes at all to the core ruleset, you just have to make sure you stick to the spell list of a single class.
The problem comes if you're going to let characters take the top blasting/debuff spells from the wizard list and combine them with the top healing/buff spells from the cleric list. I've had lots of players propose this with ostensibly pure motives but the invariable result is an OP character, because class spell lists aren't balanced against the synergies that can occur when characters can mix-and-match across them.
You can say, well, as DM I'll just police that and make sure only weak-to-average spells are chosen by the character, but then you've taken on a lot of work for yourself while generating disenchantment on the part of the player at your heavy-handedness. Or you can try something like the prohibited-school mechanic, though one thing I've learned over the years is that players are very good at identifying the prohibited schools that won't get in the way of their OP ideas. Or you can do something like the mystic theurge and make sure the character is always behind a "pure" caster of either type, but then even if you end up with something balanced overall the character won't be the best at anything so you'll end up with player complaints.
So the question you should ask is first, in a party-based game is this a solution in search of a problem? Then if you decide it is a genuine problem, what mechanical safeguards are you going to put in place to ensure your new class doesn't unintentionally become the game's top caster?
