Ok, did some more tests, got some more weird results.
I tested on my favorite subject/victim, Manglemaw.
Used a level 70 yellow crafted 2H mighty weapon.
My sheet attacks per second using this weapon were 1.44.
De-equipped my Wastes boots and my skull grasp to minimize WW damage. No damage-buffing legendary gems or passives (no Gogok, Trapped, Ruthless, etc.)
Using the DD’s rune (not automatic since no Wastes 6), and Ambo’s + Lamentation in the cube, I removed the first 25% of the target’s life in 111 seconds.
Switching to the Blood Funnel rune, I removed the 2nd 25% of the target’s life in 118 seconds. This surprised me. I had expected to see a significant fall off in damage, because I figured that essentially, using the DD rune would allow Rend stacking to hit the cap of 2, whereas with another rune, I would be hitting a cap of 1.44 (my aps). And if this was the case, I would have expected the 2nd 25% of life to take about 154 seconds to remove. Instead, the small difference between 111 and 118 seconds can probably be attributed to just a little bit of extra damage from DD hits.
For the 3rd 25% of the target’s life, I de-equipped Lamentation in the cube, and returned to the DD rune. This quarter took 220 seconds to remove, which is about 2x as long as the first quarter took, with Lamentation equipped.
This is kind of strange. Lamentation was doubling my damage, despite my 2H WW theoretically not being able to stack Rend 2x/second. This, combined with the fact that DDs didn’t seem to be giving me any extra Rend damage, despite clearly being able to proc Rend, would suggest to me that WWing with a 2H weapon is proccing Rend considerably more frequently than is shown on your sheet APS.
Update: 1.44 APS gives you 27 frames per attack using WW. To get Rend 2x/second, you just need to stay above 1.2904 APS. So any 2H setup you use just needs to reach this point to get the max value out of Lamentation.
Update #2: and since DD’s proc Rend (within 9 yards), and have a quicker breakpoint (you only need 1.00 APS to get a 30 frame brakepoint), you may only need 1.00 APS to get max value from Lamentation.