generally its best to just run a single alt for simplicity sake.
however if you find your HO alt is still not enough, you may find you need another alternator. also its generally easiest to just run the alternators in parallel versus running each one for seperate duties on the car.
hallowpoint: you can find a place to mount an alternator just about anywhere in any car. for some reason people think you need to have the other alternator sitting next to the original. im sure if your crafty enough you can make a bracket that sits it next to the ac compressor, power steering pump, etc etc. just depends on good you are with your hands in making a bracket for the situation. im sure there are cars out that that just simply dont have the room to fit anything else under the hood, but most cars ive seen have it, just not right next to the original alt.