Piano clefs with different dynamics

The grand staff is two clefs. What you did is make two different instruments. The solution that works best is simply placing the dynamic on the staff, clicking it to open up the dynamic in the inspector, and flip the dynamic range from "part" to "staff."

As Ispil says, and as has been responded to your previous post about this (which wasn't even 1h old!) https://musescore.org/en/node/271247#comment-829586

Creating different instruments to be able to make the dynamics different on the staves is unnecessary. You can assign a dynamic to a staff (rather than a part) in the inspector. You can modify several dynamics at once by selecting several dynamics and making the change in the inspector. This is also true of hairpins.

There are lots of ways to customise things to your liking One way is to star with a Grand Stave (piano), enter a series of notes and apply a different dynamic to each and to make a set for each stave. Right-click on a dynamic and "Select All Similar Elements", go to inspector and change Dynamic Range value to "Stave". Now go to the bass notes, right-click on each in dynamic in turn and change Text Properties / Offset, Vertical to -2. Now, unsure that you have "Custom" palettes selected and the Dynamics palette opened. One by ons, [Ctrl][Shift] drag your new dynamics to the palette and then sort them out neatly. You now have some "normal" dynamics that apply to both staves and a set of Treble and Bass clef dynamics.

Further playing - adjust the Vertical values in Text Properties, have a set of normal and a set of louder dynamics (easier to hear when playing back to edit a score, for instance), use the normal dynamics for general piano work and the higher-up dynamics for the voice lines in a piano/vocal score (gets them out of the way of the lyrics).

Sorry to pick up on this ancient post, but can I ask - does a "stave" level dynamic apply to both staves within a piano grand staff? No--one's really stated this one way or another.

In the enclosed, I specified "part" level "ppp" on the upper tenor and "fff" on the lower tenor, yet on playback they both play "fff". So the part level dynamic ignores the voice it is attached to. Any way around this apart from splitting the tenor lines onto two separate staves? (which in this case I have good reason not to want to.)