Depends how complicated you want it. You could have a simple relay or a full on electronic controled effort at 2 grand from Antares or Adverc.

Plus a split charge won't help it start, it will only help by running any auxilliary electrical items off that second battery like beacon and work lamps, stopping you flattening your cranking battery..

A simple way of doubling the amps up on your starting battery is fit a second battery and connect the positive onto the positive of the origional battery and the negative to the engine block. This will double the cranking amps and will be constantly charged by the alternator. Ford used to fit this on Transits as a special vehicle option for things like ambulances etc...