Stacking Question

if i have a Bortaks bone cesta ( which is Death Magic +1 20%) and I have a Deadly Cesta tha has a Death Magic +1 mod on it, is it possible, that sometimes i will cast a +2 death magic spell?

You can't get a Deadly Cesta with Death Magic +1.

Apart from what savio pointed out, yes, there's a chance

it is impossible but it works like anything else, you have (roughly) a 36% chance to get +1 death magic and 4% chance for +2.
However you are lying about that weapon, and it is impossible (unless youre thinking of a staff with +1 death magic 20% and a cesta with +1 20%, in which case 3 hands ftw)

If it worked like the other 20%s stacking, it'd be 32% +1 and 4% +2.

the (roughly) indicates the innacuracy of my memory