I have had fair luck in the past dealing with my own buddy-sour horses, but that vice can be a tough nut to crack.
I ended up leaving one stalled and turning the other out, leaving the one in the barn alone in the dark to shriek his lungs out. Usually, after the "tantrum" stage was over, all would be quiet, and life would go on. The worse of the two required a little more of the "walk away from buddy, praise and rest away, work near buddy, walk away, praise and rest away..." formula. Now they're pretty good, more jealous if they're the one left in the barn! I can ride either one without a peep or a fuss, but while I'm out, the other gelding will still whinny periodically.
Another thing that helped us is not to put them out in the same paddocks. We're lucky enough to have the land and fences to accomodate that arrangement, and even in adjoining paddocks they are less herd-bound than when they shared a turnout.
It can be fixed, or improved, anyway, but, as I said, it can be tough, and depending on the horse, may require a little more know-how than the average amateur.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown