If you find two common words which are compatible (as opposed to not compatible with each other) with each other and still available for registration. I'd first look at archive.org to see if it had been registered before. I'd use Bodis's domain check to see if it was a primary or secondary (banned domain) and I'd use MajesticSEO to see if there were any useful backlinks. If all looked reasonable, ie especially not banned, I would spring the regfee for a gamble. But it is very rare for me to even look for such a domain, and even rarer to actually find one available.
Compatible or Not Compatible, example. I'd say BlackGuns.com the words are compatible, but PurpleGuns.com the words are not compatible. Maybe not the best example, but the best I could come up with on the spur of the moment.