The previous advice is all very good and effective; please also be aware you can easily map an entire user directory to another volume/drive, and while logged in as a different Admin user, use the UUID target feature found in System Prefs->Users & Groups-> [Username]; Right-Click for ‘Advanced Options’; select the new target destination of the (already copied) target user folder.
This can be very helpful if you have an infrequent user, but whose directory – especially massive temp and cache files, massive Mail, Photos, etc – takes up precious space on your fastest drive; that user suffers from slower disk speeds, but is presumably already not a high priority user.
@james.cutler is also correct that Symbolic Links are the only proper way to push individual directories onto another volume/disk, but I do want to caution that if you edit audio/music/movies/video, be sure you create the current project directories on your local fast media, else you will be screaming at rendering times. Instead of mapping your entire Music/Movies/Photos directories (which are frequently also the default locations for AV apps projects), consider instead linking only the largest and least-speed-dependent of these subdirectories.
As for directory structure on the second drive/volume, consider, e.g., [otherdiskname]/Users/[same short username]/Movies, so in case you ever add another user, keeping these directories separate is easier; plus you can more easily set user permissions on these external directories to better respect privacy; also, if you ever have to do a migration, Migration Assistant will then better manage the affair, as will Time Machine, creating a unified User backup, as opposed to backups of separate volumes. I wish we had better, easier control of Hard Links, as this would then become invisible even to us as the user (as is the case with your Fusion drive).
Further, if you have a HUGE Applications or Utilities folder, or infrequently used but very space-consuming individual apps, these can also be placed in subdirectories, which are then also linked to separate volumes/drives; the same is true for /Library/Application Support/[app name], but it’s getting more complicated as macOS tightens up security.
The days when you could easily edit FSTAB and map Applications, VM, et al, to other volumes, is sadly long past. Even with my vintage Mac Pro, stuffed internally with 14 individual drives (4 HDD, 10 SSD), I still use, and since the earliest days of System 7, and especially with OS X, always kept my System, Applications & Utilities, and User directories as individual volumes. If my system gets whacked, I need only restore my system (or boot from emergency clone), and do not ever suffer the tremendously long times required to restore a unified volume containing all data and applications. But for an app license here and there that needs revalidating, it’s a flawless strategy, and isn’t as complex as it seems.
If Apple ever does release a new Mac Pro, based on current pricing, there is no way in heck I’m going to be able to afford (let alone be willing to pay) their outrageous (if justified) non-upgradeable permanent SSD prices in the sizes I would require for comfortable all-internal storage; unless they shock me by allowing for a few M.2 or gawdforbid SATA connections, or enough PCIe slots (HA!) to host additional internal storage, I don’t relish the games I’m going to face with ever-tightening security and the pushback from the T2 controller.
My RR840 RAID-0, striped with a pile of relatively inexpensive SSDs can easily top 1600MBps; while it’s not the 2600MBps of all the new Samsung Polaris-controlled, RAID-0 internal storage Apple is mandating for everyone, it’s still wicked fast (compared to HDD, single SSD and esp. Fusion) and allows me to keep loving a ten year old 8 core Xeon Mac Pro, and I have pure freedom to manage my over 40TB of INTERNAL storage, before we even discuss my externals.