Gotta go with the solenoid on this one. I've tried the isolators and the relay type and have had much better (and reliable)results with the solenoid.
Cheap does not always mean not good...
The isolator type runs current through diodes, NOT good if you're in a high current situation.
To setup a dual bettery system correctly, use the same gauge wire between all devices in the chain (batteries, alternator, isolation relay and grounds)
All batteries have to be of the same type, age and size, preferably brand new.
hook the trigger wire up to an IGNITION wire, not accessory. Better yet, use an RPM activated switch to trigger the relay. You only want the batteries connected together when the engine is running. Aux ground the body, chassis, engine and battery.
and that's about it. I've used this setup with as many as 20 batteries on the stereo side and it worked flawless for years.
The real question is why do you want dual batteries. If it is to have extended play time with the key off with out running down your starting battery, good idea. If it is to supply more current to the stereo system, bad/useless idea.
If you need a better current supply for the stereo the ONLY real solution is a bigger/better alt.
Audiowizard: To me using IASCA/USAC to enforce an original comment actually discredits it some what (at least to me...) those organizations were put together as a sales devices. Therefore alot of what they preach is sales influenced. Not challenging your personal experiences, I respect experience alot more than words. If a dual isolator works for you great...but I have installed thousands of systems with hundreds using the relay and had zero problems, Of the few that did initially want a dual isolator eventually ALL switched over to the HD relay setup.
just my .02, no disrespect intended.