If your cat runs on 18 cells and the boat only has one deans plug and you have two 9 cell packs, you need what is called a series adapter. Its an adapter that connects to two batteries to double the voltage. They're easy to make. (look at the pics below)
Or you can buy one:
http://www3.towerhobbies.com/cgi-bin...?&I=LXNKY9&P=7

As for the charging, what charger are you using? Because the charger type depends on how many packs you can charge at a time.

By the way what 18 cell cat are you talking about? What's the name of it?