Quote Originally Posted by Sigurd Fafnesbane
It depends on what you add together.
Instead of the additive effect we normally expect you could get synergetic effects which increases the value or potent of that which is added together.
Go ask a chemist.

Two parts of this poison added with two parts of another poison could increase the effect above double. It does not become 4 but 5. Similarly two other poisons lose some of their potency and become 3 not 4.
Well, otherwise it is specified, why should we assume it is anything else than ordinary numbers?