It’s not divided, Public and Download are the default shares bound to the firmware of the SS, it’s not split. You can fill the 6TB (RAID 5 takes one drive out for parity) on the Public folder or the download folder.
it is not like the partitions on the computers harddrive: both “drives” can use the full capacity of the Sharespace.
So if you just want to use one with the 8 TB (with RAID 5 will result approx. 6 TB) just ignore or unmap the other and fill only one. You will enjoy the full capacity.