Google usually favors domain name exact matches.
For example, if your domain is mailonline.com, when you search "mail online", it would recognize the similarity between the two and likely put mailonline.com in the front.
However, mailonline.com wouldn't work the same for the search query "email online", and would rank lower even if you put all the keywords related to "email online" there.
In this case, if you buy both domains, and redirect one to the other, would it help with results from the two queries?