With no RAID controller, and only doing Windows Dynamic Volumes,the answer to how it's handled is in part answered by how you configure the volume: You can make it stripe (the key feature of RAID 0) or concatenate (fill one then the other).
If you have a RAID controller you can configure a RAID-0 to stripe across both drives and Windows 10 will consider it a single drive presented by the RAID controller. In this case, the drives fill up at the same rate, each containing half of every file you write to the RAID, and (the main benefit for RAID-0s) this scenario will give you approximately double the read and write speeds of a single drive.
Some details and questions (some because you're talking SSDs):
- Do you actually have a RAID controller? It's dedicated to making
this as transparent as possible to Windows.
- Do you particularly value SSD longevity (they live longer than most
- If yes, does Windows 10 include support for TRIM or other SSD
longevity techniques in Dynamic Volumes (Windows 7 did not)? I don't
have this answer.
- Does your hypothetical RAID chipset and the drivers for it support