Just bought an iMac with 2 solid state drives. How do I use it?

Solution 1:

Many iMacs and other Macs have what is called a "Fusion Drive". These are made up of a small-capacity SSD (solid state disk) and a large-capacity HDD (mechanical hard drive).

Originally, they would be formatted in such a way that the OS would present them as one continuous volume. The point was to gain the benefit of the fast SSD and the large storage of the HDD.

The previous owner has replaced the mechanical drive with a new large SSD.

Each separate volume can have an OS installed on it; otherwise, they present themselves as sub-folders of the /Volumes folder. You can store whatever files on them you like.