If the EX2 is in either RAID0 or RAID1 mode, then all you will see is one disk.
RAID0 means combine the two disks into one virtual disk of the overall total size. So if you have two 4TB disks, your EX2 will show a single 8TB disk as it spans both together under the hood.
RAID1 means that the content is mirrored (saved on both disks at the same time - the two will be identical). So if you have two 4TB disks in RAID1 mode then you will only see a single 4TB disk, but under the hood both disks will contain identical files and folders. The idea is if one of your disks physically fails, then all your data is still available on the other one and the failed one can just be replaced, at which point the device should automatically copy your data back onto it to maintain the mirroring.
It’s only if your device is in JBOD mode (“Just a bunch of disks”) that you will see two drives. As it’s name suggests, JBOD just treats the disks independently and shows both of them separately and there is no spanning or mirroring. So if you set up your shares all on one drive, you can end up with one drive entirely full whilst the other sits empty.
Note that if you do change the mode of the device, all of the drives involved will be formatted and all data on them will be erased in the process.