The definition of the word is "misstep" whether to stutter or to trip. It is more a style than a technique and, thus, is difficult to place an exercize to. However, try this.
Take a 6 step basico (side-front-front-side-close-step [forward or back depending upon whether dancing man/woman]). Now dance steps 1-2-1-4-5, for example. This is side-front-back-side-close). Now dance it in the rhythm of slow-slow-and-slow-slow (where and is a quick step). Now dance it in the rhythm of slow-slow-and-slow-and. If you now add the original last step #6 (forward/back depending) as a final slow, you get a nice traspie. Dance the basic 6, then the first exercize, the basic 6, the third exercize, and finish with the basic 6. It gives you a wonderful feeling of traspie.
Of course, there are others (combinations). Try some. Bonne chance et bienvenue aux DF.
For whatever it's worth, an BsAs teacher told me that by moving (not trying to do tango/milonga steps) on milonga music and on the toes (or at least the front part of the foot), the traspie will come by itself. She does that when practising/warming-up at home.
For me, the steps I would use in Milonga are a subset of what I would use in Tango (basically, there is nothing new in Milonga that I couldn't do in Tango). However, the "feel" of Milonga is much different, (more playful, maybe less serious). Also, I do not do pauses in Milonga, (like I would in Tango). I step on every beat, and occasionally do "quick steps" or traspies.
Vals for me is similar in "feel" to Tango, but the rhythm is different (3 quick steps for one slow step), which modifies (and I suppose restricts) which steps I can do to it.
Note of caution...the OP did not ask for milonga or vals. He/She asked about traspie. Though the posts are good, they seem to imply that traspie only occurs in milonga. Re dchester's post, it is good to note also that, though there are similarities between all 3 dances, each is danced with its own unique technique and style, and is not really just a slower or quicker version of tango.
The exercize that I offered above is just something that we have used in the learning/developing of traspie regardless of the dance.
You are welcomed. Glad it helped. It will also work, in the very same manner, for milonga. Know that, of course, the syncopations will take on a quicker (almost Salsa feel) movement. This will require a correct and controlled balance (weight on the flat of the balls of the feet), and a well maintained center (body position).
Not certain what you mean by "the" double step. This would indicate that you are referring to a specific step. Traspie, or the "misstep", is, in kind, a double step, but is better thought of as a contratiempo or syncopated beating of a step. Traspie is not only a step; it is also a style, and isn't always done with just a double step. Sometimes, it is found/created in other movements.