You need to space the boards to allow for expansion/contraction with weather changes. How much depends on how dry the wood is to begin with, what type of wood it is, what type of finish will be on it, etc, but about 1/2" seems about right to me if it's dry pine. When you install the battens, nail them to only one board, so the other board is free to move behind the batten when it changes size.
Pine is not a very good choice for outdoor siding, as it's life will only be about 20 years or so at best. You should consider a wood that holds up better outdoors like cedar, fir, white oak, cypress, etc. which would hold up many times as long.