I'd suspect a mineral deficiency or perhaps external parasites. Do they have a varied diet? They need a wide variety of seeds, fruits, vegetables and grains. Google 'how to make chop for birds'. It's a really easy way to get variety into them. Vetafarm do a spray to kill external parasites. It's called 'Avian Insect Liquidator'.
Or if they aren't getting along they may be plucking due to stress. Just because they are a pair doesn't mean they will get along.
Doesn’t appear to be plucking to me..can u get closer pics of their feathers? Idk if ur able to hold urs..but I’m leaning towards nutrition issue or parasites as well..what do u feed and what brand?