You can train twice per day, but you need to make sure your recovery and nutrition is 100% on point. It's VERY easy to over train when training twice per day. The average person with a 9-5 job, family, kids etc. wouldn't be able to do it.
I personally only train once per day. I have a lot of other prioritizes that take the rest of my time during the day. I am of the opinion that if you train properly, one workout per day is all that is needed.