Agreed!  All staves in then Piano group should be equally effected by the sustain marking under the bottom Piano staff.  Please advise.

MIDI sustain pedal events are of type Continuous Controller #64, set to 1-127 for "pedal press" and to 0 for "release pedal". Continuous Controllers are a MIDI channel message and thus affect all notes on the channel where they get sent. You can set up (or modify existing) expressions to have the playback effect to send these controller values.

Usually, both piano staves would share a common MIDI channel. Thus, it makes no difference to which staff a pedal expression is assigned to. The requests' part "the sustain pedal marks...should be made to effect the other staves of the piano part as well" is already in effect.

The request would only make sense if the piano staves are assigned to different MIDI channels. You can do that in Finale, of course, but in that case, you'd better assign non-printed pedal expressions to the upper stave seperately which send on the upper staff's proper channel. What you request means basically is to set a MIDI channel for a playback effect and have it automatically sent on another channel too - a source of potentially great confusion.