You won't need a second battery to keep things up while the motor is running Lyonardo, the alternator will cover it all, however, if you want to run the stereo when the bike is parked, then a second battery may be called for. I installed a second battery in the right bag for this purpose. If you do a search you can find the post with a diagram of how it is wired so the auxiliary battery stays charged while running, but can run the sound system when parked with no draining of the main battery with the flick of a switch.
Here, I found the post, this is what I did,,
http://www.valkyrieforum.com/bbs/index.php/topic,74662.0.html