Blogging requires a decent amount of frequent efforts and contribution for content development, SEO, social media optimization, blog promotion and maintenance. If you have one blog and you are alone managing it, make sure that you only think of starting and managing a second blog if you have the time bandwidth to double your efforts.
The vital drawback of MultiBlogging is that you tend to allocate more of your time towards the newer blog as the initial stage requires more attention and efforts and this could kill your efforts on the first blog.
How to tackle such a situation: If you are keen about starting a second successful blog, you should go for it but either make sure:
1. You have doubled your time and efforts (not divided)
2. Hire a writer to support your writing activities
I run several blogs and hence use both the above options to ensure that none of my older blogs are impacted with the addition of a new blog.