Latin effectively lost its dual number. It left behind some remnants, most notably duo and ambo. However, all examples or relics of the dual number in Latin I have seen are in declension. I would assume the grammatical number to be present in conjugation also, so that between unus canit and quinque canunt there would be duo canX, where the X stands for a third person dual ending.
Classical or later Latin as I know it does not really have a dual; all examples of dual forms can be practically treated as irregular plurals. This leads me to phrase my confusion as this series of questions:
- Assuming there ever was such a thing as dual conjugation, when was it lost?
- Did Proto-Indo-European ever have it to begin with?
- Are there remains of originally dual conjugation that survived in Latin, presumably as plural forms?