You have a 1 in 2 chance of pulling a cursed arrow the first time.
If you pulled a cursed arrow the first time, the second arrow has a 4 in 9 chance to be cursed. Otherwise, it's 5 in 9.
Personally I'd have resolved this as a single d10 once, and rerolled a 10 on the second arrow. I haven't done the math to know if 3d6+1d4 <16 yields the same probability though.
1.2kg of cannabis would be like a lifetime supply for me. That's a cumbersome amount of pot.