You can use only one and add a second later. If you do this, you have three options. You can add the second as JBOD and have full storage space of both drives. Or you can create a raid0 which makes both appear as one drive. Third you can create a raid1 which has the second drive mirror the first. I know in raid0 you will lose the data on the drives as it has to reformat both drives. I believe you also lose it in raid1, but I’m not 100% sure on that one.