multiple voices on the same staff

Is it possible to place two different musical "voices" on the same staff? I often see this occurring in commercial sheet music, and I need to do it in MuseScore.

If so, how can I score this using MuseScore? I've tried a couple of things, but can't figure out how to do it.

An obvious application of such a capability would be to place the melody lines of two singing voices, e.g. soprano and alto, on the same staff.

My application is somewhat different, because I am scoring for piano solos. An example situation would be when the right hand is playing four note chords in the treble staff, and the left hand is playing arpeggios up and down the keyboard, sometimes wandering into the treble staff alongside the right hand chords (like in a typical Rachmaninoff piano concerto). In this case, within the treble staff, I'd want to score the right hand chords as its own voice and the left hand arpeggio as its own voice, both on the treble staff.

Right now I'm trying to work around not knowing how to do this by scoring the lefthand arpeggios strictly on the bass staff and using the 8va lines when the notes wander far above the bass staff lines. But for many reasons, this workaround is less than ideal.

Anybody got any ideas?

Thanks