According to this story in the West Hawaii Times (free registration may be required), the County of Hawaii is considering amending its Charter to form another Planning Commission, and splitting jurisdiction between a Leeward commission and a Windward commission.
No one has challenged the validity under state law of a county having more than a single planning commission (even though Maui County for some time has has three — one for each populated island within the county), and state law can be read to require that each county have a single commission. Jesse Souki analyzed the issue in this post on his Hawaii Land Use Law blog. Also, see his comment below.