Lots of people like the idea of blogging. Sometimes it's difficult to get started but once you do it's easy to get hooked. Blog post ideas lead to more blog post ideas. Your blog is doing well and before you know it you find yourself wondering if you should start yet another blog.
I know this type of person exists because I happen to be one. I also know that I am nothing compared to many professional bloggers (people that make a living off of blogging).
Why even start a separate blog? Why not just blog about anything you feel like on one blog?
You could. No one is stopping you. However, a blog that covers a wide range of topics is going to have a hard time attracting traffic. If you want people to read your blog, your posts must remain focused. Magazines sell because they cater to a specific audience, not all audiences. A blog is no different.
Ok, so you're staying focused and you're going to start another blog. You're tired of only blogging about new recipes you've tried and now you want to blog about funny cat pictures you've seen. Now what?
Before you hit the start new blog button (which is surprisingly easy to do), stop yourself and consider for a moment if you could talk about your new topic over an extended period of time. One really good post idea does not make a blog.
Now consider that if you start a new blog you are literally doubling your post writing commitments. If you want to keep up with your current blog and get a new blog started, you need to double your efforts.
If this still doesn't sound like it will be an issue for you, go for it! The important thing is not to overextend yourself. If your current blog is doing well and you want it to continue doing well you can't neglect it for newer, shinier objects.
Yes, it is possible to adjust how frequently you post and not have a blog suffer. How much flexibility you have depends on the topic you're writing about. Non-Techie Blogging, for example, covers "evergreen" content. In other words, the material will be applicable for a long period of time. I could schedule this post to appear at any point in the year and in will still be relavant. Evergreen content has more flexibility. Topics that are time dependent are less flexible. Politics or media would be a good example of this. If you wait too long, the issue you're discussing may no longer be relavant.
Point being, you need to know your audience and you need to be clear about your blogging goals if you want to increase readership. Having more blogs can help alleviate boredom or writer's block but it's also more work.