Maybe you don't understand how the charging system of your car works. The alternator does not directly power your electrical system. The electrical system throughout your car (including the various electrical components of your car stereo) is powered by your battery. The alternator in turn, changes engine power to electrical power to keep your battery charged so that you can start your car again. The dimming that you are experiencing is the amps ******* power out of the battery and for whatever reason, the alternator can't keep up. This could be several things but most likely:
1) Alt. not putting out enough current (not in this case, as you have 150A alternator)
2) Old/Weak battery (I assume that the battery you have is in good condition)
3) Wiring not adequate to handle the additional current (probably your case)
Now, back to the issue at hand. Two batteries will NOT fix your dimming problem. It's simple- a battery is a load on a charging system(in this case, the alternator). Add another and it's twice the load. Add another and it's three times the load. If anything it could get worse by adding additional batteries.
Before you hook up two batteries, try upgrading your stock wiring. Start by upgrading the gauge sizes for the "magic three":
Bat to ground
Alt to bat
Alt to ground
More often than not, this will totally fix your dimming problem. The shear fact that the dimming did not stop even when you upgraded your alternator makes me believe that the stock wiring is not capable of handling this additional current. Try this and let us know what happens.
Just for reference, I am running approximately 500 or so watts on my stock alternator and an Optima redtop. My lights were dimming as well, so before I invested in anything else, I upgraded the magic three. Now no dimming whatsoever whether at 2000 RPMs or at 800.