If all your drives are RD2 (Root-Data-Data) partitioned, then they all show as "shared".
Technically speaking one drive on either controller can be used as a spare on both. Why?
If Any drive fails, it fails on both nodes. This causes a reconstruction on both nodes since the drive is partitioned between both. If there is a WHOLE (unpartitioned) drive, it is automatically partitioned to match what is there and reconstruction occurs on all three partitions (Root/Data/Data).
If you have a mix of RD2 and whole SSDs (unpartitioned), then you should leave at least one unpartitioned drive per controller.