It's called a "pecking order". The bird doing all of the mounting is the dominant male, and at some point established dominance (ie. kicked the snot out of) the non-breeding bird. Yes, IMO (I don't know of any actual genetic studies to confirm [8D] ) toms grouped together like this, with nearly identical body statures, are from the same hatch...

In addition, in those situations, typically the dominant bird will do all of the strutting/mounting and the sub-dominant will be vocal. You see a very similar situation with a dominant strutter and very vocal jakes, which never strut or if they do receive a very serious arse-kickin by their more mature "leader"...

TIP: If you can bust these two apart on the roost the night before your hunt, both birds may become extremely vocal. Oftentimes the sub-dominant tom will come running in (silently) to your calling, trying to get a "piece of the action" while the dominant tom is busy tending to his own ladies. If you have a good sized piece of property, this is a well-proven tactic on grouped-up toms.

I had a pair of toms very similar to this a few years ago, first shooting the slightly larger strutting tom, then a week later dropping the second (extremely vocal) gobbler to finish my season...

Good luck!

S&R