Have you considered using Finale’s feature Part Voicing?

In the dialog box Manage Parts, expand the view by clicking “Edit Part Definition >>”, then click “Edit Voicing…”.

I could be wrong here, but it seems to me that Part Voicing can do, what you need.

Are you asking about the printed page or playback or both.

The printed page is quite easy. Unfortunately, I’m away from my computer for a few days but I have some examples that make it easy to see. I can post these when I get back.

The printed page. The only solution I've come up with is to create a new staff with cello in layer 1 and bass in layer 2 and hide the originals. But if I make a change or correction, I'd have to do it in 2 places, which is never a great idea.

Finale 25.5

Windows 7

Printed page? Very easy. Combined Cello/Bass parts were the norm through Schubert.

I take it that you are neither a cellist nor bassist—that’s ok, most aren’t. I was both during my playing years and have seen this hundreds of times. 

Unless the parts are complex to the point that separate staffs are required, use Layers to put both on the same staff. Use VC or V.C. to indicate the cello (Violincello) and CB or C.B to indicate the bass (Contrabass). German scores use KB or K.B.  When the divisi is no longer needed, Tutti indicates that everyone is playing the same part. Don’t use the English DB (Doublebass).

If you need to put the basses stems up and cello stems down, don’t worry, the players will understand what goes where. Writing an octave passage with the CB on top and VC below so that both are playing Unison not unusual—it only looks like an octave passage; again, the players know.

If composing complex passages that would be confusing on the same staff, write on separate staves and use Show/Hide as needed.