I’ve been toying with this idea on-and-off. (No doubt somebody has already done something like this.)
Perhaps create a third class—I’ll call in Munchkin in lieu of a better name—which would be a Warrior/Spellcaster built using the fractional base bonuses (UA p. 73).
On the other hand, you could just use the straight multiclass rules. Maybe—at the least—create one-level class similar to the appentice level rules to allow a PC to be a Warrior/Spellcaster at first level.
Generic Spellcasters get access to all the cleric, druid, and wizard spells. While, in this system, I wouldn’t make the normal divine/arcane distinction, I have thought of making a different distinction. Spells from the wizard spell lists might be difficult to acquire and might come with an additional cost.
Other UA rules I’d consider using: