My Imperial assassin character decided to uses poisons in this playthrough and I've had fun experimenting with different poisons. This is one of the combinations I've made with weakness to poison.

http://steamcommunity.com/sharedfiles/filedetails/?id=413385084

I think it works like this though I maybe wrong.

1) Multiply the weakness to poison with the weakness to fire

0.28 * 42 = 11.76

2) Add the product to the weakness to fire

42 + 11.76 = 53.76

Then...

http://steamcommunity.com/sharedfiles/filedetails/?id=413385203

3) Multiply the enchantment damage by the new weakness to fire

12 * 0.53 = 6.36

4) Add the extra damage on

6.36 + 12 = 18.36

I like to take out my calculator when playing Skyrim sometimes just to seem how numbers add up.