Alright, so you can have multiple blogs, and no, others cannot see the other blogs you own unless you link them. Every blog is like a different person. I have a blog on tumblr that i use to reblog things, then i have my little side blog, for all the things that hit the sweet spot of my little side.

They can't see them unless you set it up so they can. Separate is good but so is together depends on if you have fans that specifically don't like one type or stuff you don't want seen professionally.
Me I have a separate ask blog and a separate blog for reblogging randomness