Might be desease, might also be due to stress. See Actually, gouramies are like bettas, they are somewhat territorial (among the males). This feature doesn't show in the lfs as there are too many of their kinds to fight for, sometimes over 50 in a 2ft tank. However, when brought home, there are less competitors and when the conditions are right, the gouramies will start thinking of... well... sex! which is when they start to dominate each other. Sometimes, it's really hard to tell between the sex when choosing gouramis at lfs unless you're experienced enough.

As in both of your case, i think that since just one of the two died of, growing thin. and the other remains healthy as a bull, this should minimise the possiblity of desease, though i'm not saying that possibility should be eleminated. However, in your scenario, i think you have bought 2 males, and when they tried to compete for production right, the stronger one just won! and the loser will freak and eventually died due to intense stress. i never experience it with my gouramies, but went through it with my swordtail, the defeated male just freaked. when i remove it from the tank to another small quarentine tank, he grew back his colour and size within a short period of time.