Our GM Paul’s multiclassing house rule for spellcasters in Pathfinder, in case you find it of use.
The problem: Multiclassing in D&D works fine for the martial characters and skill-based characters, the abilities of the various classes stack together well to make a stronger character. The rules are very punitive for primary spellcasters. None of the spellcasting classes build on each other and none of them stack well with the abilities of any other class. Various fixes have been attempted for this in the game (prestige classes like Mystic Theurge and Eldritch Knight etc., Practiced Spellcaster feat, variant class features, and so on), but they all seem kind of specific and kludgy to me. Why can’t I just make a fighter/sorcerer and have it be effective? These rules are intended to address that.
The rule: To take advantage of this rule, you must have at least one level in a primary spellcasting class (bard, cleric, druid, sorcerer, wizard). For every two class levels you possess that are not in that class, you advance your spells per day, effective caster level, and spells known as if you had advanced one level in the primary class. You do not gain any other benefits (like channel energy, wild shape, extra feats, class powers, etc). However, you may not take your total caster level higher than double what it would otherwise be.
You may use multiple caster classes to feed into each other (a Cleric5/Sorcerer4 would cast as a 7th level cleric and a 6th level sorcerer, but would only have channel energy and domain powers of a 5th level cleric, and bloodline arcana and bloodline powers of a 4th level sorcerer).
Any prestige class that adds caster levels to a primary spellcasting class (such as Arcane Archer or Dragon Disciple) only adds the caster levels specifically listed, you can’t count it (for the class it adds to) for the purposes of this rule.
This rule replaces hybrid classes like Mystic Theurge and feats like Practiced Spellcaster, so those are no longer available.
A Fighter2/Cleric3 would cast spells as a 4th level cleric (but channel energy and have the domain powers of a 3rd level cleric).
A Ranger4/Wizard2 would cast spells as a 4th level wizard (but have school powers of a 2nd level wizard). Ranger is not a primary spellcasting class, so ranger spells would be unaffected.
A Fighter6/Druid1 would cast spells as a 2nd level druid (but have class features as a 1st level druid), because you can only double caster level at most.
A Paladin4/Sorcerer1/DragonDisciple2 (Dragon Disciple adds +1 caster level to Sorcerer) would cast as a 4th level sorcerer (1 for the sorcerer level + 1 for Dragon Disciple + 2 for the 4 paladin levels). Because Dragon Disciple adds to the bloodline abilities, the character would have bloodline powers as a 3rd level sorcerer. The paladin spells would still be cast as a 4th level paladin.
A Rogue3/Sorcerer1/Wizard2/ArcaneTrickster2 (Arcane Trickster adds +2 caster levels to sorcerer) casts as a Sorcerer5 (1 from sorcerer + 2 from Arcane Trickster +2 from the other 5 levels) and as a Wizard4 (2 from wizard + 3 from the other 6 levels, but maxes out at 4 because you can only double the caster level).