splitting grand staff

I am trying to transcribe a Vivaldi cello concerto for guitar ensemble.

I am having trouble splitting the grand staff for the harpsichord part into two parts (treble clef for guitar and bass clef for bass guitar).

I need to either separate them into two parts or be able to select multiple measure within one clef to copy and paste. So far I cant figure out how to select only all of the bass clef or treble clef.

Please advise.

Thanks

Aaron