I don't agree with your assessment of when we've used the 2 DMs. We did away at Liverpool. Did not at home. But that's neither here nor there.
While we did come away with a point at Liverpool, the system didn't work well. We played poorly for stretches, mostly as a consequence of being slower in thought and play.
Granted, because we often only try this against better teams, it could just be that it's the fact of the other team being better than normal that's the common factor and not our tactical change.
But in my view, playing with 2 DMs generally results in us playing poorer. Now when I say playing with 2 DMs, I'm talking about when we lose the 2nd AM to do so and not when a fullback or Center back joins the DM as an Auxiliary DM.
In the former, we often play poorly, in the latter our performance stays the same.