Adding ethernet ports - increase in gbps
Yes, assuming the following two details are true:


The device can utilize both ports at the same time, and the software can handle multiple connections at once. This is often not true, but for embedded systems you can do this if you design for it.
The switching network can handle it, i.e. if you have a 1Gb/s connection going to your ISP or device of concern and a router, running 2Gb/s worth of bandwidth from your computer to the router will do nothing.


For example, I work for a medical device company and we have a large embedded system running 4 10Gb/s Copper Ethernet. Now a semi custom UDP protocol and Jumbo Frames is used so it is not to crazy to manage. To take advantage of this though we leverage some Altera FPGAs to handle the data on a hardware / firmware level long before any real software touches it and the OS/Software level sees it as a single Ethernet connection so programs can take advantage.