Hi, some friends and I tried to practice this a couple days back. So I guess main takeaway points (two years later! Haha):
It's hard to practice with "imagined" disagreements or devil advocates; our group often blanked out when we dug deeper. Eg one thing we tried to discuss was organ donation after death. We all agreed that it was a good idea, and had a hard time imagining why someone wouldn't think it was a good idea.
Choice of topic is important - some lighter topics might not work that well. We tackled something lighter afterwards - "Apple products are bad". It was slowly refined to "apple products cost more than the benefit provided to the consumers, compared to other smartphones". A quick internet search pulled up the costs of an iphone vs a normal smartphone, although we realised that value also depends on the consumer. We abandoned it afterwards because it was possibly too light to find a double crux - the crux was likely to be apparent in the "problem statement". Or perhaps it could have been better seen as "If apple products (or rather, just the iphone X) cost more than the benefit to consumers, then they are bad."
We then asked our iphone user if she would get twice the amount of utility out of an iphone as compared to a standard smartphone (because that was the rough difference in price), and she said likely no.
(It rested on another assumption that moral truth is determined by humans collectively.)
We spent about 45mins to 1 hour of time defining "objective moral truth" to one another first; having a common understanding of the terms used helps facilitate the discussion.