If the 2nd alt. is big enough to carry essential loads, it can be
lighter than even the 12 lb battery & your endurance isn't battery limited.

A lot tougher to install, though.

I feel pretty safe with one Odyssey battery, and one alternator.  I've used the current battery for about 2 years, and already have the new one sitting at the hanger.  I think that will be my threshold of pain for replacement of these things. 

As for the alternator, I thought real hard about installing a second one, particularly since they only have that single pulley, and the rest of the engine has dual pulleys.  I finally gave up on the idea, because I just didn't think it was that big of a risk.  Tracy's EM-2 is hooked to a "Jumbo Super Bright LED", which is aimed at my face.  This thing will burn your retinas, so there's absolutely no way to not see it (before it blinds you).   Since the alternator only weighs a few pounds, I thought about packing a spare in the essential travel tools and supplies.  It is pretty easy to change with just a few tools.