I have a similar setup in my computer with an external drive and it's been working fine for a while.
This steps are based on rcoup's answer to this question, but a more GUI-like procedure is used.
It's been checked to work with Bionic Beaver 18.04
You might come across a few problems due to this bug. It only affects the manual mount, but not the automount proposed here.
To be allowed to do administrative operations with nautilus, you need to have the extension nautilus-admin installed
sudo apt install nautilus-admin
Then, follow these steps:
1) Open GParted, Right click the partition you want to use as Home2, Information, Copy its UUID.
2) Using Nautilus, navigate to Other locations, Computer, etc.
3) Right click the file fstab, Edit as administrator, Add the line
UUID=the-UUID-you-copied-from-GParted /mnt/Home2 ext4 defaults 0 2
Save the file
4) Navigate to Other locations, Computer, mnt.
5) Right click blank space, Open as administrator.
6) On the admin nautilus window that opens up, right click blank space, New folder, Home2.
7) On GParted click GParted, Refresh devices. Check that the mount point /mnt/Home2 shows up for your partition.
8) Still on GParted right click that partition, mount on, /mnt/Home2. There should be a key showing that the partition is mounted. Close GParted.
9) On the admin nautilus window navigate to Other locations, Computer, mnt, Home2.
10) Right-click blank space, New folder: .Private-A, Hit Ctrl-H to see hidden files.
11) Right click .Private-A, Properties, Permissions, Group: , Close admin Nautilus.
12) On regular Nautilus: Navigate to your home folder, Right click blank space, New folder: Home2.
13) Navigate to .ecryptfs (there's a symlink in your home folder).
Right click Private.sig, Copy, Right click blank space, Paste.
14) Right click the newly created file 'Private (copy).sig', Rename: Home2.sig
15) With the Text editor, create a new document containing this line:
/mnt/Home2/.Private-A /home/<your_user>/Home2 ecryptfs none 0 0
Save it as Home2.conf in your .ecryptfs folder.
16) Open Gedit and create a new text file /home/your_user/.local/bin/automount_ecryptfs.Home2 with the following content:
grep -q $MOUNT_POINT /proc/mounts
if [ $? -eq 1 ]; then
17) Navigate to this file with nautilus and right click on it, Permissions, Allow executing file as program.
18) Open the file .bashrc in your home folder, and add the lines:
# Automount additional encrypted home folder
In the dash, open 'Startup applications', Add, Name: Automount Encrypted Folder, Command: /home/your user/.local/bin/automount_ecryptfs.Home2, Save
Reboot the computer
You can do the same with the other user. Just repeat the steps from #9 on and use the name .Private-B for the lower folder instead.