The man page for systemd.mount states: "If an mount point is beneath another mount point in the file system hierarchy, a dependency between both units is created automatically."

I tested this and could not verify correct behavior 100% of the time.  This is unfortunately an intermittent bug -- sometimes the implicit dependencies appear to work right, and sometimes not.

Here is a typical attempt.  The error occurs near the bottom -- the second time a-b.mount is started, the implicit dependency a.mount is left inactive.

$ uname -a
Linux arch 3.4.6 #6 SMP PREEMPT Fri Aug 17 15:13:25 PDT 2012 x86_64 GNU/Linux
$ systemctl --version
systemd 188
arch
+PAM -LIBWRAP -AUDIT -SELINUX -IMA -SYSVINIT +LIBCRYPTSETUP
$ pwd
/etc/systemd/system
$ cat a.mount
[Unit]
Description=A

[Mount]
What=tmpfs
Where=/a
Type=tmpfs
$ cat a-b.mount
[Unit]
Description=AB

[Mount]
What=tmpfs
Where=/a/b
Type=tmpfs
$ sudo systemctl daemon-reload
$ sudo systemctl status a.mount
a.mount - A
	  Loaded: loaded (/etc/systemd/system/a.mount; static)
	  Active: inactive (dead)
	   Where: /a
	    What: tmpfs
	  CGroup: name=systemd:/system/a.mount

$ sudo systemctl status a-b.mount
a-b.mount - AB
	  Loaded: loaded (/etc/systemd/system/a-b.mount; static)
	  Active: inactive (dead)
	   Where: /a/b
	    What: tmpfs
	  CGroup: name=systemd:/system/a-b.mount

$ sudo systemctl start a-b.mount
$ sudo systemctl status a.mount
a.mount - A
	  Loaded: loaded (/etc/systemd/system/a.mount; static)
	  Active: active (mounted) since Thu, 30 Aug 2012 13:11:23 -0700; 2s ago
	   Where: /a
	    What: tmpfs
	 Process: 4016 ExecMount=/bin/mount tmpfs /a -t tmpfs (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/a.mount

$ sudo systemctl status a-b.mount
a-b.mount - AB
	  Loaded: loaded (/etc/systemd/system/a-b.mount; static)
	  Active: active (mounted) since Thu, 30 Aug 2012 13:11:23 -0700; 5s ago
	   Where: /a/b
	    What: tmpfs
	 Process: 4018 ExecMount=/bin/mount tmpfs /a/b -t tmpfs (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/a-b.mount

$ sudo systemctl stop a.mount
$ sudo systemctl status a.mount
a.mount - A
	  Loaded: loaded (/etc/systemd/system/a.mount; static)
	  Active: inactive (dead) since Thu, 30 Aug 2012 13:11:47 -0700; 3s ago
	   Where: /a
	    What: tmpfs
	 Process: 4032 ExecUnmount=/bin/umount /a (code=exited, status=0/SUCCESS)
	 Process: 4016 ExecMount=/bin/mount tmpfs /a -t tmpfs (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/a.mount

$ sudo systemctl status a-b.mount
a-b.mount - AB
	  Loaded: loaded (/etc/systemd/system/a-b.mount; static)
	  Active: inactive (dead) since Thu, 30 Aug 2012 13:11:47 -0700; 8s ago
	   Where: /a/b
	    What: tmpfs
	 Process: 4030 ExecUnmount=/bin/umount /a/b (code=exited, status=0/SUCCESS)
	 Process: 4018 ExecMount=/bin/mount tmpfs /a/b -t tmpfs (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/a-b.mount

$ sudo systemctl start a-b.mount
$ sudo systemctl status a.mount
a.mount - A
	  Loaded: loaded (/etc/systemd/system/a.mount; static)
	  Active: inactive (dead)
	   Where: /a
	    What: tmpfs
	  CGroup: name=systemd:/system/a.mount

$ sudo systemctl status a-b.mount
a-b.mount - AB
	  Loaded: loaded (/etc/systemd/system/a-b.mount; static)
	  Active: active (mounted) since Thu, 30 Aug 2012 13:12:02 -0700; 4s ago
	   Where: /a/b
	    What: tmpfs
	 Process: 4044 ExecMount=/bin/mount tmpfs /a/b -t tmpfs (code=exited, status=0/SUCCESS)
	  CGroup: name=systemd:/system/a-b.mount

$