In modern American rudimental playing it is fairly common to see Double Stops or Flat Flams. That is, notes intended to be played simultaneously with both hands — not a flam, but exactly together. Hitting in this manner produces less fundamental tone and more overtones, but it is generally quieter and less impactful than a single stroke or a flam. It is used for its differing timbre, not its volume. Other names for this type of stroke in America: Doubles, Unison, Unison Hands, Boths, Flats, Pop Flams, Popped Flams, Double Verticals, French Flams, Dead Strokes, Harmonic Flams, Double Whammy, Double Piston Strokes.
The “double stop” term comes from string instruments and refers to placing your fingers on two strings at a time. Pressing down on a string against the neck to raise the pitch is technically called stopping the string. Violins, for instance, are normally played with one finger on one string at a time, but it is possible to place fingers on two adjacent strings at the same time, doubly stopping them, and bow both at the same time. Percussion has absorbed this term for any two hits together, often in a keyboard context, but also double stop is the most correct description of the technique on battery instruments as well. The term “flat flam” is actually a pejorative description for a flam rudiment that has been incorrectly executed, such that the two independent notes have been played too close together. The failure often stems from the player not correctly setting up his/her hands at differing heights prior to the stroke. Instead of two heights, they are the same height and thus look flat. Flat flam is also commonly used to describe an intentional use of both hands together, however.
This type of stroke has only really been used intentionally on snare drum in American playing during the modern DCI era. Marching and rudimental drummers of the American past would never have played this way, and no drum books, repertoire, or solos from prior to the late 20th century ever mention such a practice. It just wasn’t done on purpose until sometime in the last 30 or 40 years. Outside of the USA, the story is a bit different.
The oldest confirmed use of a double stop type rudiment comes from Bavaria in 1781 where they used a pair of double stops called a Rücker. Bavarian military drummers would continue to play with this rudiment even after Bavaria was officially absorbed into the German Empire in 1871. An 1892 manual shows the rücker clearly used in a march. Modern sources of marching music from Bavaria do not seem to mention it, however.
Austrians also have confirmed written record of a double stop rudiment going back to 1851, when they called it a Doppelschlag. It appears in manuals until at least 1874, then seems to fade out of style. Manuals from 1889 and 1911 do not show it, and a modern packet of marching music from Salzburg in 2003 doesn’t have any reference to the rudiment, either.
John Galm of the University of Colorado asserts in a 1965 paper that the Dutch have a rudiment called De Dubbele Slagen that is distinct from a flam or a charge stroke and must therefore be a double stop. 19th century Dutch manuals seem only to show flams and charge strokes, but no definite double stops. The descriptions and notation are not very clear, which leaves open some interpretation. Modern Dutch sources also fail to show a definite double stop. So, perhaps I am looking at the wrong sources (there are several I cannot find from the early to mid-20th century), but I can neither confirm or deny the existence of a double stop for certain.
Friedrich Deisenroth asserts in a 1968 book that the Prussians had a double stop rudiment that he calls a Zusammenschlag and it appears that this is distinct from flams or charge strokes as well. As with the Dutch, I find no reference to this in Prussian or German historical manuals. I have, admittedly, not seen all that many and so, again, I can neither confirm or deny the existence of this double stop rudiment.
Susan Sandman wrote a paper in the 1970s on the Philidor collection of French music from 1705 and asserts that the rudiment Retraite is a double stop during this period. Other scholars claim that the notation refers to flams and that there is no reason to assume they are something else. French manuals from the mid-18th century and onward to the present make no reference to any double stop rudiment under any name. Once again, the existence of the retraite is difficult to confirm or deny.
Deisenroth was a music officer in the German military and Galm and Sandman were both professors of music at universities, publishing in peer-reviewed journals, so I hesitate to question any of their authority outright. Whether or not these last three unconfirmed double stops exist, the practice was certainly common in some parts of German-speaking Europe from at least 1781-1892, and probably much longer outside the written record. If you believe Sandman and include our modern flat flam, the double stop has been a rudiment somewhere in the world for over 300 years. In either case, it is an interesting idea that is clearly out there in use, but rarely appears on a rudiment sheet.