Here is a
video of my level 25 Warrior fighting corudoth for 35 minutes.
This video is a bit unusual, as I broke my normal rule of being consistent with buffs. I did 3 casts of Cog Boost (40% Haste), as I was mostly checking max hit.
There were 987 main hand swing attempts and 336 offhand swing attempts over 2139 seconds. My haste would be 62% for 1530 seconds (Cog Boost was 8 minutes 30 seconds duration at my level), and 22% for the rest. The average haste assuming a 50/50 split is 62 + 22 = 84 / 2 = 42% haste. Since the haste wasn't a 50/50 split, 1530 / 2139 = 0.715 * 62 = 44% Haste on average.
1. 336 Offhand Crush Attempts [30 Delay / 1.44 Haste = One attack Every 2.08 Seconds]
2. 987 Primary Slash Attempts [40 Delay / 1.44 Haste = One attack Every 2.77 Seconds]
3. Fight was 2139 seconds long
2139 Seconds / 2.08 Seconds = 1028 Offhand Crush Attempts * ((130 Dual Wield Skill Level + 25 Player Level) / 475) = ~335 Predicted Secondary Crush Attempts on average.
2139 Seconds / 2.77 Seconds = 772 Primary Slash Attempts * (1 + ((130 Double Attack Skill + 25 Player Level) / 500)) = ~1011 Predicted Primary Slash Attempts on average.
Both of the predicted numbers above track with my log results. Also looking at my logs, I didn't see any crush attempts that occured within the same second.
So at least at level 25, it doesn't look like double attack triggers on offhand. Also it looks like 475 is the correct "MaxSkill" value for warriors, as the wiki suggests.