Not meaning to be argumentative, so let me apologize in advance if this seems so:
Going with one axle brakes may be perfectly legal and work very well for you - maybe. But again, considering the quite modest cost, *why not* go for as much braking capability as is reasonably possible for your rig? You may never need it, towing with that big dually . . . but then again, being able to stop even quicker in an emergency might well make the difference - maybe. Also, emergency braking in on surfaces with lower coefficients of friction, wet, icy, snow, etc. it is handy not to have your trailer attempt to pass you up. Having all wheels with brakes back there is not a bad idea. Did I mention it doesn't cost much? :c)
bumper
Disclaimer: I might be biased on all of this, being a former motorcycle cop and accident investigator I've seen my share.