A few ways:

Move one part to layer 2. Implode Music.

Move one part to layer 2. Show Active Layer Only to hide music in destination staff. Drag the other staff into it.

Use JW Staff Polyphony.

When using any method, check to be sure that courtesy accidentals, slurs, articulations, etc. copied correctly.

Thanks, Mike, but I would be very greatfull if you could detail some more:

What is to implode music? Under which menu? If I move both staffs, source and destination to layer 2, when I move the source staff to the destination wouldn't it overwrite it? What is JW staff polyphony? 

Again, thanks a lot for your answer!

Amit

OK, first question (and a very important one!)

Are you using the full Finale, or one of the daughter products: PrintMusic, SongWriter, or NotePad?

I'm using full finale 2014. 

Thanks!

Amit

I would do it "the long way" because I have found that the shortcuts and plug-in make some mistakes.

Let's, for now, assume you have only two staves: flute and violin.

1. Change the violin staff to layer two. Select the staff, and use Selection tool> right click> Move/Copy Layers...> Move Layer one to Layer two

2. Document menu> View Active Layer Only

3. If it isn't already selected, switch to layer two. Your layer one notes in the flute staff should not be showing.

4. Drag the violin staff (which should still be showing) up into the "empty" flute staff.

5. DESELECT Document menu> View Active Layer Only 

6. Erase old violin staff.

Hi mike. That was very clear and really works...

Thanks a lot!!!

Glad I could help!