Would it be a good idea? The product im marketing is around health and fitness, but i also want to add cooking and food under the same blog/domain seeing how eating healthy is around fitness but the thing is they are totally different products being promoted.
To me, this makes perfect sense and is really just a positioning issue. If you position the site/blog as dealing with overall health, then you can have sections for fitness, eating, and perhaps more (e.g., helping the disease aspect of health.) There should be no confusion for your audience. Quite honestly, when I read your subject line, I thought that you might be looking to combine two totally dissimilar products or services, which might have gotten a different response from me. I think that you are actually picking related or offshoot niches and that is a strategy that often works well for companies.
The way im doing it is by clickbank as an affiliate, so i was going try and do it as a review of the "product"
If it adds value to your existing viewers, I'd say go for it. Your niche's make sense to be together. The issue is going to be to do everything well. In my opinion, there is nothing worse than an old or stale section of a website.