Whether you need dual sense voltage sensitive relay depends on whether the auxiliary battery will need additional charging. As the auxiliary battery will most likely at a constant higher discharged state due to lots of accessories usage like stereo or other things while the engine is switched off. This will then lead to a shorter life span of the auxiliary battery unless it is charged externally on a regular basis. To rectify the situation, usually a secondary charging source will be installed and connected to the auxiliary battery, say a solar battery panel, or a dedicated battery charger.
The Single Sense Voltage Sensitive Relay only allows the main battery to be charged first, and the auxiliary battery will only be charged once the main battery is fully charged.
On the other hand, a Dual Sense Voltage Sensitive Relay will allow both batteries to be charged as long as either one of the battery is fully charged. This give additional flexibility to user on how to manage the batteries and battery bank efficiently.