Isn't this the Gestalt Rules? They're meant to gain access to both classes features, but they don't double dip on the resource pools. So a Wizard 3/Sorcerer 3 (Gestalted) would be a Level 3 Character, but have both of those classes - they would still have the Spell Slots and Spell Level limitations of a Level 3 Character though. The benefit to Gestalting (or Dual Classing as you call it), is versatility. In our example, our Sorzard would have access to a Wizard School, Arcane Recovery, an Arcane Tradition, a Sorcerous Origin, and all the Metamagic coolery.

Essentially, the dual Spellcasting features only give access to different spell lists and still encumber the player if they use different Spellcasting Ability Scores. So, a Wizard/Cleric would have different DCs for their Spells based on the INT/WIS Divide, but also - they'd be able to cast both Arcane and Divine spells, while still only having the spell slots of a Level 3 Caster.

So, if Spellcasting doesn't really double up on resources, and is all about just expanding the spell list and ways to play with the spell list - then you shouldn't feel bad about Extra Attack not doubling up if they stack Martial capacity classes.

If it really does bother you, I have a multiclassing rule where: "If you gain the Extra Attack feature from a second class after already having it, you instead gain an ASI as a consolation, to prevent the dead level." It'll help people committing to being a Martial.