In Prelude №1 by Villa-Lobos sometimes it's required to play 2 notes with the thumb, e.g.:
There's also a variation on this:
What's the difference between the two in terms of playing?
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In the first case, the two notes should be struck simultaneously. (So I personally just use some two fingers, I don't remember which exactly — but striking them both with the thumb just sounds too forceful. For instance, the middle part of Villa-Lobos Prelude 2 is a great place to strike the two strings with the thumb simultaneously, because the forcefulness fits there really well.)
In the second case, you should first play the low E, and only a little moment after that strike the 5th string. One after the other. That's good to play with the thumb only, because it can comfortably strike first the 6th string and then sort of "slide" (just continuing the downward movement) into the 5th string which is then struck as well.