Quite literally 'split' charge, as oppose to 'sequential'
Your volt meter could be used to read the batteries independently when the engine is off, but as soon as you're charging, the two batteries are connected together, and so you're charging - and therefore reading the voltage of - both simultaneously.
Probably not a lot of benefit in monitoring both / switching between them, as the major concern should be how well you're charging them, and the voltage on the main battery when cranking, but if you like fitting switches or gauges, go for it!