FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 196824
This article provides the steps to share a physical interface between multiple VDOMs.

The requirements are:
  • Management access to Global configuration.
  • A physical interface from any VDOM connected to a L3 device in trunk mode.
  • Use of VLAN interfaces for each VDOM.

1) Log into the FortiGate with an admin user with access to Global resources.

2) Go to Global settings > Network > Interfaces.

3) Click on 'Create New' > Interfaces:
a. The type of the interface must be VLAN.

b. Select a physical interface from the Interface drop-down menu.  It can be any interface from any VDOM.

c. Set a VLAN ID for this new VLAN.

d. Under Virtual Domain, select the VDOM that will use this VLAN.

Click on the '+' button next to the green arrow icon and the physical interface will show the newly created VLAN interfaces and their respective VDOMs.