Yes, boxers tend to get along the best with other boxers. It doesn't take them long to learn to recognise them either
and they'll often seek each other out at parks, ignoring other breeds of dogs when there's another boxer to play with.
So, if you're considering a second dog, then another boxer is a very good match. Opposite sexes are generally the most successful combination though (providing they're spayed/neutered, of course). Two females can be a disaster, and two males is usually OK (better neutered though).
There are many many members here with two or more boxers. And most of the time it's a very successful combination. They keep each other well entertained
Btw: Please take the time now to introduce yourself to the community in the introductions forum. Per the site rules, that is supposed to be the first post you make on the site, and before participating to the rest of the forums.