Actually three USB ports, the one in front panel is USB-3, and two lan ports.
From the specs it seems to be based on the Marvell Armada 370, identical to the one found in the DNS-327L.
Knowing manufactures, the DNS-340L must be very similar to the DNS-327L, probably with only extra SATA multipliers and the second lan active (the Armada 370 has dual SATA ports and two network interfaces builtin).
If you end up having one you can install 'ffp' on it and then telnet or ssh it. I would then appreciate if you could post the output of some commands, such as cpuinfo, meminfo, dmesg output...
Don't know what the D-Link fw does with the second lan port. The Armada-370 has single core, the Armada-XP is multiple core.
It could be setup in parallel/bonding with the other, duplicating throughput (not many users would use that, as it would require 10Gbps physical network devices), be setup as a redundant/spare of the first port, for high availability systems, or acting as a gateway between two different networks, as you suggests. Everything very exotic for me or the average home user.
Sincerely,
Dmitri Passaita