I like this concept. Spell slot access is nice, but spell access is king. And you're giving spell slots that are slightly higher than a 10/10 split would get, but just under what a hypothetical 14/14 split would be.Going along with my rules I've figured out how to tweak the spell slot progression:
Double Caster Examples:
- Character level determines hp, proficiency, cantrip damage, and, if a double caster combination (e.g. Sorcerer/Warlock), spell slots per day (based on the most favorable class treating Warlocks as a full caster, see examples at end)
- Multiclass level determines class features, spells known/prepared, and, if a caster/non caster combination (e.g. Barbarian/Druid), spell slots per day
- Warlock double casters treat Mystic Arcanum spells as additional spells known which require spell slots and can be upcast regularly
A character level 20 Eldritch Knight/Arcane Trickster gains spell slots as a level 20 third caster (Eldritch Knight or Arcane Trickster)
A character level 20 Eldritch Knight/Paladin gains spell slots as a level 20 half caster (Paladin or Ranger)
A character level 20 Cleric/Paladin gains spell slots as a level 20 full caster (Bard, Cleric, Druid, Sorcerer, or Wizard)
A character level 20 Paladin/Warlock gains spell slots as a level 20 full caster (instead of pact magic progression)
Caster/Non Caster Examples (I used 14/14 as the example level 20 equivalency here):
A character level 20 Eldritch Knight/Assassin gains spell slots as a level 14 third caster
A character level 20 Four Elements Monk/Sorcerer gains spell slots as a level 14 full caster
A character level 20 Barbarian/Warlock gains spell slots as a level 14 Warlock (standard pact magic progression)
I think 14th level max is probably right around the sweet spot for this; I'd probably have their progression stop at levels 2,5,8,12,15,18 (+1 every other level).
I'd say my only other thing is that I'd like to see double martial concepts get something for having 2 copies of Extra Attack. I don't know what exactly, I just think they need something to make them more compelling.