Should I blog on different domains for personal and 2 different products?

Look at it backwards: Which brand do you want to build/strengthen? Is your goal to say "I'm good at what I do, and here are the things I am working on," or is your goal to say "This is X, a new company. Here are our experiences building it."

I think if you decide this first, then the choice of how to blog will be easier. If you want to do both, then that will be the hardest way because you will probably need to keep two/three sites, with fresh content for all of them.

As to whether or not merge products under one domain/blog, that really should be aligned with your branding strategy: Are the two products openly marketed under one company? If so, then one blog for both would be feasible.

I like to take the opposite perspective, though with a similar conclusion. Instead of "what do I want to say?" the question is "who is my audience, and what do they want to hear?"

If your audience is for a specific product, and they want to hear news about it (new features, success stories, etc.) then that is one blog. If you put something about another product or a something personal on this blog, does your audience care?

If you want to write an article about starting a business or something personal, your intended audience is probably different than people looking around for news about your product.

As with any writing, the first and last question you ask yourself is "who is my audience?"

I would suggest doing the following:

Sign up for a free account with Google Apps. Register your domain and use a sub-domain for each product.

For example

productA.mysite.com - Blog for product A productB.mysite.com - Blog for product B mysite.com - your site

By doing this, you still keep them separate, which I suggest you do because in my opinion the key to successful blogging is "specializing". This allows people to know you are an authority on "essay writing" and they'll keep coming back for that. However, when promoting your product blogs, people will see the association between product A and mysite.com and there may be positive spill-over effects (more views of your blog or even views of the product B blog).

The good news is that to test this out it is FREE. Once you have the mysite.com domain, a combination of Google Apps (Standard) and Blogger will let you test this idea out without needing much technical know-how.

Good luck!