This is an easy upgrade--cheapest way is to get 2nd battery identical to 1st, but not the best idea to combine a new and old battery. Best way is 2 new batteries of the same mfg date. Car batteries not good for this, need deep-cycle type. Alternatively, for longest life, get 6-volt Trojan batteries and wire them in series for 12 volt. Otherwise, like sc3283 said, above--being sure to wire them in parallel like he says.
You might be able to also get 2 battery boxes on the same tray to keep them clean.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died