252 Atk Choice Band Hustle Durant Iron Head vs. 252 HP / 252+ Def Weezing: 165-195 (49.4 - 58.3%) -- 66.4% chance to 2HKO after Black Sludge recovery
Probability that Durant hits both Iron Heads with high damage roll
(80% * 80% * 66.4%) = 42.496% Durant wins
If we ignore flinches and crits, the above part is correct and Weezing wins over half the time, but this already a borderline case, so I think it's fair to take these variables into account. Looking at what happens if Durant gets a low roll...
Probability that Durant hits both Iron Heads with low damage roll, but crits on one or both
(80% * 80% * 6.25% * 93.75% * 33.6%) + (80% * 80% * 93.75% * 6.25% * 33.6%) + (80% * 80% * 6.25% * 6.25% * 33.6%) = 1.26% + 1.26% + 0.084% = 2.604% Durant wins
Probability that Durant hits both Iron Heads with low damage roll, no crits, second Iron Head is a flinch
(80% * 80% * 93.75% * 93.75% * 33.6% * 30% chance to flinch) = 5.67% Durant wins
Probability that Durant hits both Iron Heads with low damage roll, no crits, second Iron Head is not a flinch
(80% * 80% * 93.75% * 93.75% * 33.6% * 70%) = 13.23% Durant loses
Probability that Durant misses one or both of its Iron Heads
(20% * 20%) + (20% * 80%) + (80% + 20%) = 36% Durant loses
42.496% + 13.23% + 2.604% + 5.67% + 36% = 100%
42.496% + 2.604% + 5.67% = 50.77% chance of Durant winning.
(this doesn't account for some highly unlikely winning scenarios like Durant missing first Iron Head but flinching the next two, etc, point is that Durant wins in over 50% of scenarios)