Seeking a clean way to mount a directory on one HFS+ drive as a directory on another such drive on same machine
To mount a hard drive on a custom mountpoint:


  You can do this with diskutil mount and the -mountPoint option.
  From the man page:
  
  
    mount [readOnly] [-mountPoint path] device
    
    Mount a single volume.  If readOnly is specified, then the file system is mounted read-only, even if the volume's underlying file
    system and/or device and/or media supports writing; even the
    super-user may not write to it; this is the same as the rdonly option
    to mount (8).  If a -mountPoint is specified, then that path, rather
    than the standard path of /Volumes/VolumeName, will be used as the
    view into the volume file con- tent; a directory at that path must
    already exist.
    
    From man diskutil, section 'Verbs'.
  
  
  The syntax to mount to a custom mountpoint is as follows:

diskutil mount -mountPoint /path/to/custom/mountpoint [volume (identifier/UUID/label)]

  
  Bear in mind that /path/to/custom/mountpoint must be a
  directory, just like with mount, and that your
  identifier/UUID/label are specific to the volume (i.e. /dev/diskXsY
  not /dev/diskX). Mounting to a custom mountpoint cannot be done with
  diskutil mountDisk, and only works with a single volume at a time.


From my answer to How to mount disk by UUID or LABEL in OS X El Capitan.

I've tested this by "mounting" a USB stick on an external hard drive. As an example,

$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh SSD           499.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS HDD-1T                  999.8 GB   disk1s2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *31.6 GB    disk2
   1:               Windows_NTFS USB-32                  31.6 GB    disk2s1


As you can see, /dev/disk2s1, or USB-32 is the name of the USB stick I am going to mount on the hard drive, /dev/disk1s2, or HDD-1T. To do so, make use of the -mountPoint option. But first, make sure there is a landing directory to mount onto:

$ mkdir /Volumes/HDD-1T/mountpoint


And now, to mount:

$ diskutil mount -mountPoint /Volumes/HDD-1T/mountpoint /dev/disk1s2


The mounted disk will appear in the following locations: /Volumes,  and /Volumes/HDD-1T/mountpoint as USB-32 and mountpoint respectively.

Be sure to unmount (diskutil umount) your USB-32 equivalent before remounting on your HDD-1T equivalent.