The "Best 50/50 Multiclass?" thread got me thinking, can the rule that you are limited to your spell selection as per the level of each class be changed to limited to the same level that determines your multiclass spell slots and still be balanced (i.e. using the multiclass table for spell slots and prep levels)?
The reason I ask is that except for Class 1 level 1 or 2 (or sometimes rarely 3) / Class 2 level 10 types of PC are the only types I have ever seen. Even in martial only multiclass PCs, I have never seen Cleric 6 / Wizard 7.
This makes sense for special class abilities (i.e. not double the number of special abilities over a single classed PC), but I wonder about spellcasting. Is it really a good multiclassing rule if virtually nobody ever takes a Cleric 6 / Druid 6? (and yes, there might be a few players out there that probably do, but we are basically taking a small handful out of millions of players). People view these multiple spell caster multiclass builds as totally undesirable.
The main question is: Is the gain of higher level prepped spells unbalanced with that of a single class PC?
Note: the PC is still limited to prepping the number of spells as per the current multiclassing rules. The sole difference is what level of spells the PC can prep.
Let's compare a Cleric 6 / Druid 6 to a Cleric 12 (assuming 20 Wisdom). The PC loses the ability to wear metal armor (i.e. loses 2 to 6 AC), loses 6 preps of Clerics spells, loses 2 level 3 and 2 level 4 autoprep domain spells, loses an ASI, is limited to destroying CR 1/2 instead of CR 2 undead, loses a Divine Domain feature (like +D8 to weapon damage), loses Divine Intervention and a cantrip. The PC gains 3 or 4 cantrips, gains the ability to prep 10 Druid spells of levels 1 to 6, gains either the ability to wild shape into a level CR 1/2 creature and 2 level 1 and 2 level 2 autoprep circle spells, or the ability to wild shape into a level CR 2 creature as a bonus action, and if circle druid, gains travel through certain difficult terrain and saves vs. magical plants.
Most of the low level abilities gained are not very impressive (low level wild shape hit points and attacks in high level encounters), it's mostly the druid spells that really matter. And the PC gave up quite a bit to gain those (i.e. the lower AC and the higher level Clerical abilities and higher level domain spells). I don't see this as particularly unbalanced. The PC gains a lot of versatility, but loses quite a bit as well. The multiclass PC is still limited to the same number of spell slots as the single class PC, s/he just gets a much larger pool of spells to choose from when prepping (and does gain 4 additional prep slots).
Cleric 6 / Wizard 6 over Cleric 12 would gain significant spell casting abilities and AC. A significant cost would be the MAD of having good Wis, good Int, and then fairly low Dex and Con, and Cha/Str typically being dump stats (e.g. starting 16 Int instead of 10 Int would be 7 point buy points lost elsewhere). There are some higher level Cleric abilities lost like 2 level 3 and 2 level 4 autoprep Domain spells and the Divine Domain feature, but the gain of casting Wizard spells would typically make up for those. There is a slight difference since the bonus to hit with spells and the DCs of spells would tend to be 1 or 2 lower for at least one of the classes, the AC would probably be 1 lower, an ASI is lost (and MAD ASI occurs), and the PC would probably have 18 fewer hit points on average.
Or alternatively, Cleric 6 / Wizard 6 over Wizard 12 would lose about 6 hit points (MAD Con being 2 less), lose an ASI (and MAD ASI occurs), lose an Arcane Tradition feature, loses 6 prepped Wizard spells, gain 1 or 2 points of AC, and gains 10 prepped Cleric spells, gains 2 level 1 and 2 level 2 domain autoprep spells, and a few minor Cleric abilities (like turning minor undead).
Ranger 6 / Paladin 6 has the same spell casting as current multiclassing rules.
Cleric 6 / Ranger 6 over Cleric 12 gains an attack per round, gains ~6 hit points, loses an ASI, loses casting 1 5th and 1 6th level spell per day, loses casting 6th level spells at all, loses level 3 and 4 Domain spells, loses Divine Domain features (i.e. a little less combat damage), loses a cantrip, gains Fighting Style, gains Ranger Archtype, gains some minor Ranger abilities, gains level 1 and 2 prepped Ranger spells.
All in all, none of this seems particularly unbalanced to me. Two full spell casting classes that cast totally different types of spells seem to gain the most, but those are also the classes that tend to experience MAD.
If you think that this is unbalanced, please give a build example of imbalance and don't just state an opinion. This isn't the type of house rule evaluation which can be analyzed with opinion. It needs examples.
Edit: Changed domain and circle spell levels from caster level to spell level to avoid confusion.
