No, this is not possible, two NAS cannot be connected together and operated as one system, for this you need a hard disk extension, such as the DX517. It is compatible with the DS918+ and offers an additional 5 storage spaces (this is also the maximum that a DS918+ can use, the 9 in the DS918 stands for a maximum of 9 disks and the 18 stands for the year of release).
Synology DX517 Expansion Unit