If I am correct, I believe that this is also the method others use behind the scenes.  In front of things, they simply have a "layer number" that specifies which segment gets the notes you are entering, so you simply change the layer number and continue entering, with the notes going on the other segment.

D. Michael McIntyre wrote:
On Monday 24 August 2009, Gustaf Borgström wrote:

  
I understand there no option for adding layers in Rosegarden, so I would
like to propose this feature to the community.
    

We've had something good enough to get the basic job done for a long time.  
Sorry I have let this sit here so long, but we've been extremely busy with 
development lately.

I don't have any tutorials more focused on this one specific issue currently 
available, but you can get some idea how to work with overlapping voices on 
the same staff reading this one:

http://rosegardenmusic.com/tutorials/supplemental/piano/index.html

What we have is not quite like the "layers" idea that you and others before 
you have proposed, but this is pretty much how it's all going to work in the 
future, because this was the way to get the thing done. We talked about all 
kinds of big and complicated ideas for the perfect solution to the underlying 
issue, and then one day (a few years ago) somebody realized that if you had 
two segments on the same track at the same time, the notation from both of 
them would already appear on the same staff.

That set the tone for how the reset of it would eventually unfold, following 
the path of least resistance.  There were a lot of problems trying to work 
this way in the beginning.  Now that we have expanding height tracks and a few 
helpful functions, there are fewer problems working this way.

I have to say that in the future, this will always be done with independent 
overlapping segments, rather than "layers" within a single segment, but there 
is a great deal of room to improve how all of this currently works, and make 
it both more friendly and make the back-end mechanism more transparent.

For example, at the top of my list, I would like it to be easy to create a 
parallel segment "right here" and put notes in it, without having to go out to 
the main window and draw one, then open a new edit view to include it.

We will not be doing any of that for the upcoming release, however, and any 
new enhancements in this area are too far in the future to offer any idea when 
they might ever happen.  Maybe for the release after next.  It is not 
impossible.  This is an area I am very interested in concentrating on in the 
near future, but neither is this whole issue my most immediate personal 
priority.

I am more concerned with the way it is completely impossible to deal with 
anacrusis properly in a piece that has a series of repeats.  Our concept of 
what a "bar" is has always been broken.  After that is fixed, better support 
for working with overlapping parts is a very high priority.

But keep in mind I mean better support, rather than support.  I wish I had a 
tutorial explaining how you can already do SATB the somewhat hard way.  It 
will hopefully get easier in the future, but it is by no means impossible now.

Well, I don't think so anyway.  Honestly, I'm not 100% certain about how 
lyrics come into play with this, and all the parts singing different lyrics.

I do know it's no problem running horn music with I and III on one staff, II 
and IV on one staff.  Aspects of it may be annoying, but it can be done.