Let's say we have poisons A, B, and C with strength A<B<C. So if someone takes A then B then C they will die, correct? What happens if they take B then A then C? Does C antidote both? Also, does the Witch know what the highest strength of poison the wizard can make is?Spoiler for If so...
Theres a problem with this solution:
About CaptainEd's proposed modified competition; we can't/shouldn't alter the test in any way, its part of the brainteaser.
About the strength of the wizard's poison:
All i can say, because it's all I know is:
Wizard's strongest poison will always be weaker than witch's strongest poison.
Wizard's weakest poison will always be stronger than witch's weakest poison.
By how much? we don't really know exactly but the difference should be big.
Yoruichi's answer is close, it just need to go a different direction.