Multiple VDOMs using the same physical interface

In the FortiOS 5.6 release notes, there is a new VDOM interface sharing feature (launched in 5.6.1) :

https://docs.fortinet.com/uploaded/files/3602/fortigate-whats-new-56.pdf

Page 238

Create a virtual switch that allows multiple VDOMs to use the same physical interface or VLAN (436206)

This feature allows multiple VDOMs to access the same network or the Internet using the same physical interface rather than requiring each VDOM to have its own Internet-facing interface.

However try as I might I cannot get the feature to work. The main issue is that the VDOM link endpoints refuse to be assigned to the software switch. I have tried this on a couple of FGT units, both running 5.6.3.

Has anybody tried using this feature?

Regards,

Pete