You were going in the right direction in wanting a grand staff instrument. Here's how to make one.
0. Optional: Experiment in a test file.
1. Go to
Home > Instruments > Edit Instruments > - VDL Template 7.0a > Marimbas. Choose a marimba and select it. Make a copy of it by clicking
New Instrument, then
Yes.
2. Change
Name in dialogs to whatever you'd like. (Could append with "~GRAND", or whatever.) Click
OK.
3. Add the new instrument to the Marimbas Family list by looking in
Instruments not in ensemble and scrolling to the one you just made. Select it, click
Add to Family.
4. With the new instrument now in the Family, select it, and click
Edit Instrument.
5. Under
Notation Options > Type of staff > Number of staves, select
2. The
Clef for second staff should default to Bass. Click
OK.
6.
Close the Edit Instruments dialog.
7. Add new grand staff marimba to score.
I also quickly looked to see what else was out there and found
Cross-Staff Notes function is not working and
Sibelius 8: Tutorial No. 20: Cross-Staff Notes. Feel free to
Ggl for more.
Let me know how it goes.