Would it be possible to tell the client, "allocate 100GB to X, 300GB to Y, 100GB to Z" and have the files downloaded and verified in the right order?
It depends on the client, but gocoin, bcoin and Bitcoin Core (i've tried all of them in past) doesn't have such feature.
You can also use Storage Spaces on Windows to create logical volumes out of multiple disk, or alternatively RAID0 would work as well.
RAID0 only works with two disks that must be the same size, and be attached to the same RAID controller that often requires the two disks to be almost identical models, if I recall correctly.
These days using RAID controller is optional. Many motherboard these days provide RAID support on software level, which AFAIK have inferior performance and occasionally unstable.
--snip--
It is technically possible to tell a client to allocate specific amounts of storage space to different drives and have the blockchain files downloaded and verified in the correct order. This could be done by specifying the file paths for each drive when configuring the client, and then using a script or program to ensure that the correct files are being downloaded and verified on each drive.
Which client support this feature? Can you name any example?