Adding another alternator would require some pulley and belt work, just as adding power steering or air conditioning. It could be done, and has been done.
Electrically, I don't see a problem with a separate 12-volt system. But maybe could use a 6-volt to 12-volt inverter to power the electronics, if they do not draw too much current. Remember that the 6-volt side would require more than double the amperage of the 12-volt side, so you would need adequate wiring and fuse or breaker.
I had a 12-volt pusher fan and a jury-rigged second battery to get the 12 volts. I replaced the pusher fan with a 6-volt unit, and deleted the second battery. Then I added an inverter for cellphones, GPS devices, etc. So far, so good.