I'm writing a montuno two piano ensemble and primo has its melody in the right hand completely duplicated by the left, an octave lower. How could this be abbreviated in the score? I can't find any piano examples and I'm not very familiar with orchestral sheet music, but I guess it's something like basso continuo?
I come across this in practice/teaching material, where for example the Primo (student) is the same melody in both hands while the Secondo (teacher) is providing the harmony.
There (although aimed at students, not performers) the convention seems to be to write both hands in full and note the octave shift with 8va alta/bassa, in some modern engravings on the clef itself.
See for example Diabelli Op. 149 ("Melodische Übungsstücke"): https://imslp.org/wiki/28_Melodische_%C3%9Cbungsst%C3%BCcke%2C_Op.149_(Diabelli%2C_Anton)
Or from some student material I own (don't do this in any non-practice piece, it's considered non-standard notation):