You can use the National Luna dual battery gauge with other dual battery systems . All the features work except joining the batteries together. I had the National Luna intelligent solenoid system system but it would not work in the summer heat. I switched to a 500 amp Blue Sea solenoid and have not had any issues with heat. The only thing missing was a gauge.
I cut off the plug on the solenoid end and separated the wires. The red and black wires are the main battery and the white with red and black stripes are the aux battery. I joined the ground wires together and attached to a chassis ground and the hots to the batteries. Works like a charm.