Tone mark modifier characters in Unicode (U+02E*) often use the 'ccmp' OpenType feature to combine marks:
˩ ˥ ⇒ ˩˥ (˩˥)
˥ ˩ ⇒ ˥˩ (˥˩)
˦ ˥ ⇒ ˦˥ (˦˥)
˥ ˦ ⇒ ˥˦ (˥˦)
˧ ˥ ⇒ ˧˥ (˧˥)
˥ ˧ ⇒ ˥˧ (˥˧)