Hi

There is nothing called legal or illegal in studying. You can study as much as you want and whatever you want because it depends on you ultimately if you can simultaneously study two courses together. The timelines of both the courses might crash sometimes so you will have to take care of that.

No if you want to do regular arts hons and NIOS science hs together it isn't illegal. But you have to think if it's doable!

Hope this answers your query!