When you have octaves in one hand with an 8va above the top octave notes you obviously play the bottom octave notes up an octave too.

What do you do when you have a two octave gap with one note written in each hand but joined by a line as if they were written for one hand (hope that makes sense) and 8va only above the top line? Do you play both hands up an octave or just the right-hand so that there is effectively three octaves between them?

(I don't suppose anyone has the piece but you never know - it's the piano accompaniment for a trombone piece called "Les Marches du Kiosque" by a contemporary French composer whose name I have momentarily forgotten).

I *think* the 8va only applies to notes on the top stave, even if the notes are joined together.

That's what I thought but then I wondered why bother joining them together.

Maybe to show the melodic line? (Say, if there are 2 voices maybe?)

There's not really a melodic line. It's just quavers from Bb up to Bb and then back down again in both hands while the trombone has a long held note. Earlier on in the piece there is the same thing in octaves for the right-hand only while the left-hand has a bar's rest.

Which sounds better?

Neither
I don't like the piece that much but if I had to chose I think probably putting both hands up an octave as three octaves apart sounds very skeletal (there are no notes between).