Maybe I'm old school, but for most runners (especially non-elites) I would not recommend "double threshold."
I think with supershoes/super spikes perhaps the training has evolved to allow more "double threshold" days more often...but I think it is also something for elites that might be on PEDs. Like "double threshold" would be a really good idea if one was on PEDs for sure.
For most people, it's a matter of getting up consistent higher mileage (higher volume) so they can simply just do a longer (single) Threshold or Tempo session in one go. Instead of 8 x 1km they can do 10 x 1km or instead of 4 x 1-mile they do 6 x 1-mile etc.
Maybe someone can fill me on the "the science" of why Double Threshold is "better" exactly? (or why you think so)?.......but basically I just see it right now along the lines of "crash training" or going for a "big super-compensation" (i.e. you train really hard sometimes, and you adapt to the newfound stress and you get better!).
The other concept/shift would be the idea of doing a higher % of your weekly volume at a "moderate intensity level" of 80-90% Max HR (Threshold to Tempo) instead of trying to hammer out more Vo2max or super high intensity training over 95%. So the "benefits" come more from dialing back from "Zone 5" or spiking 95% HR/effort (and over 4mmol values) for the sake of doing more volume of workouts between LT1-LT2 (Threshold to Tempo, lower lactate levels but higher relative volume). Training more "like a Marathoner" instead of a "1500m runner"....even if your target events are 1500m-10km.
But fitness and training gains usually come from a sequence of workouts over time ( a blend of Tempos, Long Runs, Strides, Vo2max, Weekly volume etc.) so generally no "single workout type" can be definitively credited for all the fitness gains. One single workout day (if it is too hard) can certainly break or injury you though....