Problem with the solenoid being hooked to the ignition switch is when you do run your tunes etc. when the vehicle is off, and lower your aux battery, then turn your switch to start your car and automatically bridge your batteries the drained aux battery will pull down the starter battery. Also if you are running enough accessories to exceed your stator's output, you will drain BOTH batteries while you're running.
We've had a lot of success with dual battery kits using Smart Isolators. They will monitor both batteries, only bridging the two if the main (starter) battery is topped off. And will isolate the two if it detects that the system voltage (while the two batteries are bridged) drops, indicating that there is a net drain; i.e. consumption is exceeding supply. If it decides to isolate the two, once the main battery is back to full charge it will attempt to bridge the secondary again on some regular interval.
It's a must have for dual battery setups.