Second battery wouldn't help low compression. If the compression was low it would be a pita to start when it got cold. Having a second battery will let you crank a lot longer. It could also mean a number of other things, weak starter, bad ground, or just a weak original battery. Maybe the PO was to cheap to buy a new battery and sistered a weak one up to another marginal battery.
People do strange stuff more investigation is needed. If it were my car I'd remove it and restore everything to stock.