Similar questions have been asked before. The closest appears to be How to mount multiple directories on the same partition? where this currently accepted answer suggests that it can be in two ways:
Using four _symbolic links: in the root filesystem, pointing to four individual subtrees on the second disk, which itself is mounted once to a mount point independent of the four directories.
Using bind mounts (available since Linux 2.4) to mount four individual subtrees on the second disk to the four directories, after the second disk is first mounted to a mount point independent of the four directories.
The same answer has an interesting comment which points to this Q&A discussing pros and cons of symbolic links vs bind mounts. From this, one would conclude that symbolic links should be preferred, because they're easier to see and maintain and will cause no trouble with any existing software.
systemd has an explicit dependency configuration feature called
RequiresMountsFor. E.g. the standard Debian 8 (Jessie) uses it to make some services wait for potential mounts of
/var, which needs to be available for the services in question.
RequiresMountsFor will only work with bind mounts; not with symbolic links.
At least the Debian 8
systemd service configuration files in question are located in
/lib/systemd so they are not meant to be modified. This suggests that bind mounts should be preferred if you use a Linux distribution which introduces such
systemd dependencies. Modifying the
systemd configuration of the Linux distribution may turn system maintenance into a nightmare.
The presence of
systemd itself, on the other hand, does not constitute a pro or con point for the choice between symbolic links and bind mounts.