Hi there and welcome to the forums,
Unfortunately, your little girl has scaley face mites, which need to be treated right away in both birds. Mites are very contagious amongst birds in the same house so all birds in the vicinity should be treated with a medicine called Ivermectin, which is applied in a single drop between the wings directly on the skin.
In a few weeks, the mites should clear up. Meanwhile, you should keep everything extremely clean. Take their cage tray outside (just the tray) and wash it with hot water and vinegar, remove all perches or toys made of fabric and throw them away (mites can burrow in them and reappear later) and scrub all their toys and perches with hot water and vinegar. Wipe down the cage bars with undiluted white vinegar, it's fine to do this while your budgies are in the cage. If you do this and treat them with the medicine, they should be fine after the course of the treatment is over.
You can buy Ivermectin for birds online fairly inexpensively!
Meanwhile, welcome to the forums, I'm glad you've been reading through the site guidelines, etc. Be sure to also check out the many great budgie articles and stickies we have available to stay up to date on the very best of budgie care practices!
Keep us posted on how your girls are doing.