FormerlyHemlock
Hero
OK, so let me get this straight:
A level 1 Fighter/Mage in your system would have literally all the features of a 1st-level Fighter (including hit points, proficiencies, and class features) and all the spellcasting abilities of a 1st-level Wizard? Compared to, say, a 1st-level Fighter, who will just have the fighter abilities? I'm not a fan of excessive intraparty balance, but that seems grossly unfair. That's almost identical to a Ftr1/Wiz1 using the core multiclassing rules--a 2nd level character.
So, 300 XP later. The 1st-level Fighter becomes a 2nd-level Fighter while the Fighter/Mage stays the same, so they're kinda even. Great. Then, 300 XP later still, the hybrid character advances in both classes, gaining the exact same features that the solo Fighter just got, plus a whole bunch of new Wizard features. So once again, the hybrid character is way ahead.
Am I correct?
Yes, as far as your question goes. But another 300 XP later, the Fighter/Wizard is behind again (no 2nd level spells). For a while, the multiclassed guy is only about a level behind the single-classed guy, but the gap widens at higher levels. A 17th level single-classed character would be 13th level if multiclassed with two classes, or 10th level if three.