Connecting the batteries in parallel would allow twice as much current to go to the starter without having to modify the charging system, etc. If you connect in series, you might as well do the whole 12 volt conversion routine, because it won't be possible to put 12 volts to only the starter - - - -the whole system will get it, and things will smoke instantly. Get the battery tested by a pro who knows what he's doing! The procedure involves a full charge, and a heavy current load of 1/2 the cold cranking amp rating for 15 seconds. There's at least a strong possibility the battery is the one and only problem.
Jerry
