It sounds like you are making 2 new blogs.
- your site design is already done, just add the text
- For SEO purposes your content and links etc. will help your current domain
- Keep people on your site can help sales and brand recognition
- Need to design 2 new sites
- In SEO you are kinda competing against yourself. If you can get both of your sites to the top of rankings great - you can have 2 entries in the SE litings. If not you are halving your potential SEO power
- maintaining 4 sites to 2 sites is double the work
- Having 4 brands can confuse the marketplace. If your 4 brands are well marketed you can dominate the industry with the top 4 brands not just 2.
I would say if your company is like Coke or MTV you can do well with its own blog site. They probably do these types of things for Summer promos etc.
For a smaller brand, I would see little benefit and big downside in making a new domain.