In one of my homebrew settings I sort of allow Concentration stacking in that I have consumable magic items called Power Components. A Power Component is consumed in casting a spell, and it allows the caster to choose one method of enhancing the cast spell. One of the enhancements available is to remove the Concentration requirement of the spell. Each Power Component relates to a certain class of spells (Healing, Illusion, Fire, Plant etc.).
This might be a good idea to test the waters with. I can introduce a few consumables to try out the concentration stacking, and see how I feel about it. If I don't like it....well no more of the consumable
