I know for a fact from my experience with 4x4s that a beefy alternator and an Optima will run 6 100w off-road lights plus ground-pounding stereo plus GPS plus DVD player plus CB plus a 12,000lb winch plus electric fans, etc. etc. Only guys that want to run stuff like TV's etc. at the campsite without risking running down their starting battery run two batteries. I can't imagine any reason to have two batteries in a Spider.....
Heck, I used to run highbeams, a pair of 55w fog lights, a pair of 100w driving lights and a 4-amplifier, 11 speaker stereo system cranked all the way up on the stock alternator in a Honda CRX with an Optima. It'd do that - all at the same time - all night long.
Chris