for my taste it only one battery , pricey but well worth it IMHO .. the brand is Odyssey
there is a thread on here [use search, it might be in a pinned thread I don't remember] .. the thread shows what size battery can fit in what models allows you to up size the battery a bunch .. example my factory battery was a small lawnmower type from the factory , I now have a 925 CC Amp Odyssey in that same space. ... tons of crank power / winch power / and will last for many years .. I have 4 other Odyssey's in use in other vehicles one is about 12 years old now and still no signs of weakening and that one gets abused a bunch .
if interested be sure to shop on line as the prices vary greatly .. they can be shipped no problem and can be mounted in ANY position even upside down [mine is on it's side in the A/C ]
I agree with Gary on the Odyssey brand, I'm going on year three with mine and never a problem! Not sure where the battery is located on a 550, but there is ample room under the drivers seat for a full size battery if you put a mount/tray in.