Hi,
I dont fully understand your setup with the LAN network, it looks like you have set the LAN interface both for your internal network, and for the link network between the firewalls, it can probably be solved but usually it is a problem for the routing of the traffic, since the hosts on the other subnet gets the traffic from the firewall FW1, and when the host will send the return traffic, they will look into itīs routing table, find the default route and send it to their default gateway, which for them will be FW2.
To avoid this I think if you can find a dedicated interface on each firewall would be the best, like the DMZ interface if it is free.
Regarding your configuration I can see one problem. 
Rules:
Name | Action | Source interface | Source Network     | Dest. interface | Dest. Network          | Service
all-in     Allow     lan                      Lan_Fw_to_Fw         lan              lannet(10.104.6.0/24) all_tcpudpicmp
all-out   Allow     lan                   lannet(10.104.6.0/24)   lan             Lan_Fw_to_Fw          all_tcpudpicmp
I think should be...
Rules:
Name | Action | Source interface | Source Network     | Dest. interface | Dest. Network          | Service
all-in     Allow     lan                      10.103.6.0/24         lan              lannet(10.104.6.0/24) all_tcpudpicmp
all-out   Allow     lan                   lannet(10.104.6.0/24)   lan             10.103.6.0/24          all_tcpudpicmp