A voltage regulator provides a particular output voltage such as 20 Volts going in to the regulator and 15V (or some other voltage) coming out - a voltage regulator prevents over-voltage.
If you have two batteries and do not wish one of them to be discharged while you use the other then do not connect the battery you wish to preserve.
If you are worried about losing your radio "keep-alive" while you change batteries over you can always "squirt" 12V into the vehicle through the cigar lighter socket.
A car battery sits at about 14.2V or so when the engine is running - a little below 12V when the starter motor is turning. A starter motor will draw large amounts of current from your battery (check how fat the cable is) 50A +.
It's not clear (not clear to me) from your question what you're trying to achieve and why.
Good luck nevertheless.
Peccavi
June 2013