It is an ensemble, if we all played the same bits it would sound awful!
most of the parts are written in the treble clef, which means the harps are being played above middle C, but there are some bass bits and I assure you they are being played!
Some of us play one handed all the time, some play a bit of both (me) and some play both handed all the time, this is because we are all at different stages of competence. I have only been playing 2 years, Gail 1 year, most of the others 3-6 years and Sarah, forever!
It wouldn't fill out the sound to have more bass strings, this is a lament and should be soft and gentle.
Hope this answers you question! when Phil gets used to working his new video camera we will play some tunes with more bass bits, if it makes you happy!
Cheers, Lucy