Jeez. Thread is 3 years old and was ANSWERED.
@emnoc: using secondary IPs is just for implicitely creating routes, to avoid Reverse Path checks/drops. You can create static routes to both subnets as well to achieve this.
If the FGT is the (only) router, and the subnets do not overlap, and there's no policy allowing this traffic then there will be no traffic allowed. You can even leave out the policy altogether - implicit deny / policy 0 will take care of that.
"Kernel panic: Aiee, killing interrupt handler!"