Safety??
A backup alternator is a great safety feature, especially when mounted on engine number two. I know, I know... I suppose, if your primary alternator can't handle the total amps a second might help, but, I would first sell that one and get another that can handle the amps - very little weight gain and maybe a bit less money lost than adding alternator #2. However, if your ignition system requires external voltage, then that would be an argument for a second alternator and maybe a backup battery or two or three.
I did lose my alternator this summer. I knew I lost it because the field breaker opened, efis voltage alarms went off, and volts dropped below 12. I turned off all unnecessary electrical stuff , then turned around and flew 2 1/2 hours home on the battery. Easy peasy.
And now, for the cockpit voice recorder, "ymmv".