Kind of hard to interpret fox barks...But it seems when they just keep barking constant their is danger..namely most times you have been busted.
By the way coyotes will do this too.. Constant barking in the distance..By the way when the coyotes do this always go to the coyote pup..You would think of going to fox pups will do the same for fox... Only three times in my entire calling years have I called back a barking fox.. That was by jumping back and forth to red fox canine sounds on my Foxpro FX5...However try it next time they, coyotes or fox bark at you or you hear them in the distance.. Go to the like animals pup distress sounds...
I have heard the two barks like you said...They bark twice and shut up only for about 45 seconds and bark twice again and shut up for about 45 seconds.. I have heard this when coyotes are in the area. Always watch when all kinds of critters are acting up.. it could mean the coyotes are there.... deer walking along the roads in numbers and not in the fields,, of course that double bark of the fox with just brief moments of silence.
What I would do in this case would go to my calling coyotes scenario. You just might be surprised when a coyote shows up all of a sudden...
Finally they could be getting that urge now since they are looking for a mate.. This might occur where you hear them barkinig even before you get your stuff and move to a calling stand.. you just happen to step out of the vehicle and you hear that bark..Again it could be coyotes in the area too...
Also think of our domestic dogs when they bark... Either are protecting their turff from intruders, want food, or want companionship (mating)..
Same applies to the wild canines..