Why must each input have a separate set of decoys?

Question

If we have a mixin of 6, with two inputs.

We immediately have a total of 6 * 2 = 12 decoys

Would it not be secure enough to have the 4 decoys with the two inputs which means two key images? So we have a ring size of 6 in total?

Answer 1

Mixin 6 means a ring size of 7.

Having 2 real inputs and 5 decoys for an overall ring size of 7 is a lower level of privacy than the current implementation, which would be 2 real inputs and 12 decoys as part of two rings of size 7.

It would be worse if you were spending 6 inputs. You'd have only 1 decoy, so your privacy level would drop enormously.

Providing a consistent level of privacy no matter how many inputs are being spent is therefore an important design goal.