William Smalley was the one that was credited for creating the Romanized Popular Alphabet (RPA) to be used in the Mong/Hmong language. He used the D marker to distinguish the tone shift from M to V from the other seven tones in the Mong/Hmong language. This D tone marker is mainly used for adverbial situations, such as ntawm to ntawd, tim to tid, saum to saud, etc. To a lesser extent, it is also used in pronoun situations such as nkawm to nkawd, and in whatever other concerns that the speaker wants to lengthen the saying in trying to drag out the poetic mood of the speech. These words that end with the D sound marker begin with the M tone and end with the V tone under the same breath. I am sure Smalley had listened to countless Mong folktales and knows exactly how the introduction to those folktales is being expressed. I have been wondering why he didn't also create a sound marker for the tone shift from V to B in the introduction of those folktales. Such a marker would, of course, be used in other situations aside from telling a folktale. Regardless of whichever variety, whether Mong Leng or Hmong Der, when it comes to this introduction of a folktale, except for the vowel variation of the first word, we all say the same thing. While in English, the introduction to a fairy tale usually begins with once upon a time long ago in a far-far away land, in Mong language we say puog thaum ub of. Since Smalley didn't create one, I've used F as the sound marker for the tone shift from V to B for this of voice. This then makes the regional dialect, mostly concerning with those in Southeast Asia, have seven tones and nine sounds. The two tone-shift markers, D and F, that shift from M to V and V to B, respectively, create a different sound from that of a single tone. Is there a name for this situation of a sound that contains two different tones? If there is, what is it called?
"Combination of multiple tones under one breath" suggests a standard term: contour tone. However, that specifically refers to combining multiple levels within one syllable. No language uses "a breath" as a structural unit (we don't grammatically compute the duration of a "breath" and don't apply grammatical rules based on how long we are talking), though some linguists divide utterances into "breath groups" where we stop talking for a moment and therefore can take a breath. The phenomenon that you are talking about seems to be about the syllable, and not "a breath" which is many syllables.
Tones in Hmong and SE Asian languages in general are usually treated holistically, combining properties of phonation, pitch and duration into a single abstract "tone", and decomposing phonation, pitch and duration (where there are already changes implicit in unit-tones for the former two properties) into a sequence of two tones would be counter to the usual holistic treatment of tones for these languages.
We can't be sure from your description what phenomenon you are trying to encode, but it sounds like this is a stylistic property, not a grammatical or lexical property. There are very many style-dependent modifications of speech, and writing systems are generally do designed with the intent to capture all such distinctions. Instead, the writing system must first capture the basic contrasts of a language, and one leaves oral art to oral transmission, rather than trying to define the limits of such a variant and then reduce it to a letter. You might gain further insights into Smalley's motivation by reading his works.
I am new to this Stack Exchange so still learning how to comment and other things. The marginalization and elimination of the usage of a particular sound within a variety of a language are what I am concerned about. RPA has been great in preserving many aspects of Mong language. From the surface, at least it made the distinction from that of the Phajhauj script that Sooblwj Yaj had created, not only representing the seven tones but also including the tone-shift usage of the D marker as mentioned above. I am interested in continuing the improvement of this writing system which is based on the Latin alphabet. It would help greatly if we know what to call this tone shift. That way, when explaining to another Mong/Hmong person, we won't have to go into whether our language has 7, 8, or 9 tones.