I have raid 5 virtual disk - 2 HDD failed and i replaced them with new ones. I see both disk as online but rebuild process did not start. I assign them as global hot spare, but in the menu, option to rebuild it is grey out and i do not know how to start the rebuilding process.
You said this was a RAID 5 volume with 2 drive failures. A RAID 5 volume can only handle 1 disk failure, before the volume fails, too. Rebuilding wouldn't be an option, you would need to redeploy.