You shouldn't need to extract parts, used the Linked Parts. If you are talking about a separate part for bass and treble clefs rather than an instrument which uses the grand staff, then I suggest changing the clef for the marimba staff below each treble clef marimba to a bass clef. This would be much easier than adding a new instrument and configuring the playback controls.
If you're unsure about Linked Parts, review the Finale tutorials.
iMac i7, OS 10.8, 10.9, W7
VDL2.5.5, Finale 2010-14
Kontakt 4-5, various other soft synths