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.
Examples:
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).
Geek Related 
You could also adopt the practiced spellcaster feat that was in 3.5. I thought it was a good rule for multi-classed spellcasters.
http://dndtools.eu/feats/complete-arcane–55/practiced-spellcaster–2231/
No, these rules specifically cite that feat as something it supersedes; we’re well aware of it. That feat sucks, it is a mandatory tax on multiclassing and doesn’t help enough.
I like it. It’s very similar to what Trailblazer did with its casting multiclass rules, but way easier to implement without completely redoing all the classes.
I like your idea. I don’t like the way the system works either. I’m playing a magus, and multi-classing into any other arcane caster just doesn’t make sense unless my available spells increase. I don’t agree with non spellcasters being counted into the spellcaster’s level, but I agree with the idea overall. A level 13 Magus, Level 1 Bard, should somehow gain 1 level of spellcasting ability.
See, exactly. Why is the magus necessary at all? Only because the mutliclass spellcaster rules suck in the first place. The point of 3e style multiclassing is that you should be able to make a decent fighter/mage using multiclassing – but you can’t, and the fault of it is how spellcasting level is treated.
I hear they will be doing “3e style” multiclassing in 5e, I hope they figure out this issue. Mages get BAB bumps, other classes should get spellcasting bumps even if they don’t cast spells per se! You could even make it parallel to the save/BAB progressions.
Here’s something I’m working on.
Base spellcaster class is highest level Arcane spellcaster.
Option 1: The base spellcaster class is used to define what spellcasting level you cast at.
Option 2: sum (Spells available per level x level), whatever Arcane class is highest, is the class that is the Base Spellcasting class.
For each additional Arcane class, add the levels of those classes to the existing Base Class for determining Spell Levels available.
Example: Level 10 Magus, Level 1 Bard, Level 1 Illusionist. = Level 12 Magus spellcaster.
Restriction:
Spells made available by new arcane classes (i.e. lower levels) are only those accessible to that class alone and based on that’s class’ level. For example. A Level 1 Bard would still only get Level 1 Bard Spells added to the Magus overall allowable spells.
Same for level 1 Illusionist spells. Opposing spells cost 2 spell slots still (i.e. spells outside of the allowable Magus/Bard).
Option 3: Add up the total # of spells available per casting level.
Example:
Casting Level
1 2 3 4 5 6
Magus Level 10
5 5 4 3 1
Illusionist Level 1
1
Bard Level 1
1
Total 7 5 4 3 1 0
Each spell casting level is now purchased at a point cost equal to it’s spell level.
Sum: (Spell’s available per level x level), to purchase a higher level spell, must spend points from the sum.
I.e. a level 10 Magus, level 1 Bard cannot cast level 2 Bard Spells.
This article rocks. I like the idea a lot.
I’d recommend the Unearthed Arcana “Magic Rating” system as a simpler solution (similar overall, better/worse in ways): https://www.d20srd.org/srd/variant/magic/magicRating.htm
It’s basically adding *caster* levels together, and granting fractional caster level to fighters/etc, but not increasing spellcasting level. So a 4 Ftr/4 Wiz casts as a 4th level Wizard, but with 5th caster level. A Clr 3/Wiz 3 gets 6th caster level on all their spells, but just 3rd/3rd for what spells.
This gives a single caster level regardless of the class combinations, and doesn’t overly increase abilities.