yellow, blue, red topI 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.
I'd agree with most who recommend a higher output alternator. You could put 10 batteries in your Alfa, but with only 45 amps to charge them, they won't do you any good.And get one of the 65 or 70 amp alternators, if you want to keep to alfa parts. BTW - there are 85 amp to 130 amp bosch alternators from BMW's and MBZ's available - they're a bit big and need a different drive belt and bracket.
70 amps = about 950 watts
130 amps = about 1700 watts (13.3 volts)