We add a feat, Focused Caster, which allows a caster to concentrate on more than one spell. However, each additional concentration spell requires a Concentration check (DC equals 10 + the spell level) when cast. If it fails, all currents spells fail. If you take damage, you make a check for each concentration spell in effect, losing all if you fail any check.
With this feat, we see two concentration spells sometimes, and very rarely 3 or more attempted. It works well to keep a balance of power when it comes to spellcasters, but allow enough flexibility when desired.