Yes, a lot of us have added the second battery. Some have opted a single car battery and did away with the tiny one.A couple guys used two small battery's with an Isolator
I myself used both. I have my stock battery running the starter and I have a full size cheap Napa battery running my accessorys. In between them I use a True Utv battery Isolator.I have seen Guys put the second battery under the hood or under the drivers seat.Either way you have to fab up a mounting bracket. If you go 2 battery's then I recommend the Isolator.It automatically allows you to charge both battery's. I also put volt meters on both battery's.I keep a small 6 inch pigtail wire with me in the case one battery should fail. That will allow me to give myself a jump.
I prefer the simplicity of one big arse car battery under the drivers seat. The extra cold cranking amps coming in handy when I haven't rode the pioneer for a week or so in the winter months and I go out at 6 am and try to start it. But to each his own,I don't carry a spare tire either.
I opted for the dual small battery setup. Works great. Got a lot of help from the fine folks on this forum! Either way you choose, one big battery or 2 small batteries done properly will work great