If I have two niches that are related, such as food and drink, would I be better off combining both niches into one website,giving me more content, or separating one site for food kws and one site for drink kws to make it more focused?
How related are the topics? Wine + Cheese? If you're looking to dig very deep into each niche, it could be beneficial to split them up. In case you want to sell off 1 site, or do graphics/theme to match each one. SEO-wise it shouldn't matter too much.
Once you run the site about food and site about drinks you have all the knowledge about the niche and can use the data you already have and easily jump to bigger site which combines both. I'm not saying it's an excellent solution but that's the way I would do.
How would you stay unique?
Let your sites grow, recognize your audience, their needs and make changes until the point you have worthy, unique site.