FortiGate Question - Virtual Wire Pair for Redundancy?

So I am replacing a core switch at a client site with two new core switches to add some redundant pathing between the Fortigate HA Pair and client access switches. We're a watchguard shop so Im still new to the fortinet stuff.

Question is, I'd like each firewall in the HA pair to connect to each core switch - in the event that one switch goes down, it would have a redundant path to the second core switch via the other link. How would I go about designating a "failover" port for the LAN on the fortigate, and is this even possible? I've read about the virtual wire pair but not sure that's applicable here.

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EDIT: It appears that most of the physical interfaces are assigned as members to the LAN interface "hardware switch" - with STP turned on, is it as simple as just adding another connection to a member port?

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Thanks in advance.