Hello,
You can install a maximum of 2 x 20 TB disks (total 40 TB), but be careful about the power supply, you need to buy one with a minimum of 48W, from 16 TB upwards (2× 8TB).

Maximum number of hard disks 2 / maximum storage space that can be created
36 TB.... so your 2 x 18 TB HDs are supported.

I have 2X 14TB, works OK 18TB should work