I think there is a French term for this - Moq? (Just kidding, I don't want the bluebird forum to turn in to an "adult" website).
But seriously, the extra male could be an unpaired offspring from last year, helping out. It happens, but rarely. Usually the male will chase the youngsters away once nesting season begins. More often, young from the same year's brood might be seen helping out. But I have seen an adult female help raise another's brood.
Maybe this happens more than we think, and we just don't see it as "another helper" thinking it is the same adult unless we see them both at the same time as you have. Without name tags or a guest list, we will never really know for sure.
Bluebars

I was able to grab my camera in time to shoot the 2 males together. Not the greatest shot, but it is proof nonetheless!

Here is a link that might be useful: Our Missouri Backyard Birds

Beautiful pictures of the bluebirds. They are however not going to be safe. Putting a nest box on a tree is easy for cats, raccoons, squirrels and any other climbing critter to get/kill the bluebirds, babies, adults and eggs. Maybe you can put the boxes on a metal pole with a baffle to keep them safe.

Karen

Karen... Glad you pointed this out about the mounting situation. But did you also notice the comment under the photo???

Amazing... maybe it means we should start pairing nestboxes (yikes!)

LindaSkyview
Western NC

Thank you for the mounting tip. I've never had bluebirds before so I'm pretty ignorant. I have never seen cats, racoons or squirrels around here in the 4 years I've lived here (that's not saying they aren't around though). We have a 6 foot privacy fence that does a pretty good job of keeping out intruders. Plus all of our neighbors have rather large dogs in their backyards. There is only a small portion of the fence that isn't "guarded" by the neighbor dogs, but there are some free-range dogs that probably help deter unwanted visitors who may be looking for a meal.

I'm glad you like the photos. :)

If you have sparrows in the nest box next to your blues like the caption said under the pictures, your blues don't stand a chance. The sparrows will kill the blues. And to think I was worried about the mounting situation :(

Now, ladies, no "yikes-ing" behind you hand - we were all new to this at one point.

back2 - House Sparrows (aka HOSPs) are NOT going to be friendly with your Bluebirds. They are non-native, invasive and aggressive. They've been known to destroy Bluebird nests, eggs, young and even kill adult Bluebirds. I'm no expert, but you should discourage them from nesting anywhere near cavity nesting birds (most would say you should discourage them from nesting, PERIOD!) and check out the Bluebird site below for all the info you could ever want or need about hosting these beauties (and getting rid of unwanted guests like HOSPs). Good luck!

Here is a link that might be useful: SIALIS.org

Holy moley! I had no idea how nasty HOSP were! I just finished unpacking their nests (we also have one other birdhouse in the yard that HOSP were nesting in). I will certainly do all I can to discourage them from hanging around. I suppose I should take the whole other birdhouse down so they don't try moving back in.