Honestly, the OA2/3 on Mythic 1 handers really should apply to both hands.
With a 2hander, like the SAM, WAR, or DRG mythics, you're looking at, at most, +80% DPS from the OA2/3. It will in practice be less, because you will already have DA TA and WA, but it is an insanely large boost.
On DNC, with AM3 up, I get 40% OA2 and 20% OA3... on my main hand, and main hand only. Which means that AM3 on a 1 handed mythic for a dual wielder is, at most, half as big of a boost as it is to a 2hander, or someone not dual wielding.
Of course, that will never happen, because tha twould mean SAM isn't the best DD job, and we can't have that.