Entering a gateway at an interface will add NAT to it by default (unless you enable advanced outbound nat). Don't set a gateway at any of your LANs for the pfSense interface config. You only need firewallrules to prevent the LANs to talk to each other. A rule like this should do the trick:
At firewall>rules, LAN-tab: pass protocol any, source LAN-subnet, destination NOT LAN2-subnet, gateway default
At firewall>rules, LAN2-tab: pass protocol any, source LAN2-subnet, destination NOT LAN-subnet, gateway default
Now both LANs can get out to the internet but not to each other.