Seems like that raises a MESS of questions... It's an easy declaration to make, but implementation is bound to be a monster.
For one, it just doesn't make sense that an item that makes one stronger and an enchanted weapon shouldn't be able to work together.
For two, how do you ultimately define "stacking" if you're going so far as to assume a Belt of Giant Strength and an echanted weapon are "stacking" in some way just because they both contribute to the same activity? Do the effects of Haste and an enchanted weapon, then, "stack"? Both enhance combat ability... How about the effects of a Shield spell and Bracers of Armor? If you rule those can't stack, then how can you justify a non-magical shield "stacking", in effect, with non-magical armor?
As stated, this doesn't work at all. It breaks the whole d20 magical item system.
If you're wanting to accomplish something like this, you'll need to create a new RPG or d20 overlay that redefines how magical items work. Even then, at best, you'll alienate your players.