OK it looks like RCC_LAN is a VLAN interface. Can you confirm by doing " show full system interface RCC_LAN | grep type".
If so, this VLAN interface needs to be attached to a physical interface, such as the redundant interface.
Can you shed more light on any of the other interfaces on the FortiGate? Is there more L3 interfaces between FGT and SW? What is the nature of the "trunk" interface between the FortiGates? How does it work and how is it connected?
Now, with that said, you probably do not need the VLAN interface unless you need trunking with tagged VLAN access to a downstream switch. Given what I see here there is no tagged traffic between the FGT and the switch. If RCC_LAN is the only L3 interface on the FGT, then I suggest moving the IP configuration, including VRRP, into the Redundant interface config. Or, at the very least, assigning the RCC_LAN interface to the "SW1-SW2" interface.
config system interface
set interface "SW1-SW2"
Given your use case, I would suggest leveraging FortiGate HA with SD-WAN to satisfy your requirements.
With FortiGates in HA, you only have one configuration to manage. With SD-WAN you can put both your WAN interfaces (and GRE tunnels) into SD-WAN interface and do your load balancing/failover using SD-WAN. There would be no duplication of multicast traffic as there is only one logical device on the network.