In Blogger, you can only have one Search Description/meta description per post. And that's okay - in 2009, Google announced that meta description tags (which are what the Blogger Search Descriptions map to) aren't used as search engine ranking factors. (Neither are meta keywords, for that matter.)
So adding multiple descriptions won't help you here. What will help you is writing concise (< 155 characters), compelling description content that will help convince a user to click through to your page from the search engine results page.
If someone searches for a specific keyword, Google may bold it if it's included in the Search Description for your post. But again, that won't do anything to help you rank - it'll just help someone know that your blog post is relevant to their keyword search, and thus convince them to click.
So it's not so much search engine optimization you're after here - it's clickthrough optimization.