Multiple physical network interfaces on the same subnet may cause routing issues. Configuring multiple network devices to be on the same subnet is not supported.
The proper way to set up the CFA when using both nics is this: configure one Nic to use specific static routes and any traffic on those routes will use that nic. (You will need to add specific routes for the second Nic using System > Settings > Routes. The static routes will need to point at the next hop router on the second Nic's network.)
The other (second) Nic will use the default gateway which indicates "everything not specifically going to the other Nic will go here".