Well this all goes onto my unified Magic Power stat.Attunement is necessary for game balance, especially to help balance casters and noncasters. To remove attunement to allow 'layering' spell effects breaks the game. 'Concentration' is narrative flavor for mathematically necessary constraint.
You can only Concentrate on Levels of Spells, Attune to Magic Items, and Resist Curses equal to your Magic Power.
Base Magic Power is Charisma Score/3.