Never heard of the company "EMC Battery", but that doesn't mean its not a quality AGM style battery. I assume that since you are looking at adding an AGM battery, your existing house battery is also an AGM battery. Ideally to maximize battery bank performance having the same style (AGM, wet, etc.), size/specifications/ratings, and date is recommended. Trojan and Interstate are also a popular brand as well.
If you do camp "off the grid" frequently, increasing the size of your battery bank has it's advantages.., but if you primarily camp plugged into shore power a single battery will do just fine (no real benefit to adding a 2nd).
If you like reading detailed battery "stuff', both of these links are an interesting read: