Firstly, I'm not even sure if there's only one alternator connected to the engine in a C172-like aircraft. But if that is true, then why don't manufacturers connect multiple alternators to a single? What are the draw-backs of doing so, other than a waste of generated power?
I just want to know whether it is possible - and preferably without any major draw-backs.
Without too much increase in weight and complexity, one more alternator could together produce twice the electricity - this might actually be primarily useful for large air-liners relying more on electricity than hydraulics, like the A380 and the 787 Dreamliner. So, again, why not?