Questions about Synology DS720+

two times tow tb

That may well be, the prices for bundles are not always the cheapest solution. However, I would not recommend a 2 bay NAS at all. A later expansion will be much too expensive, it's better to take a 4 bay NAS, because you can always upgrade something internally without having to dispose of two small disks. The DS418 would be a good choice, it offers 6 TB of storage space in RAID. Synology DS418 (Seagate Ironwolf, 4 x 2 TB)