I wish there was something we could tell you to do at home for them, but this is one of those health issues that just isn't something that is going to be easy to do anything about without knowing what the issue causing it is...
However, what you do need to do and can do from home is to first separate these two birds into a totally separate room from your other birds, so that if they do have a Viral Disease you can at least try to protect your other birds from getting it by protecting them from the feathers, dander, etc. of these two birds. That would be the first thing I would do...
***Let's assume that they do in-fact have a Viral Disease like PDD...It's not necessarily fatal, and they can shed the disease, and getting them through this is going to be all about keeping the nourished and stress-free...So, you said that they are sitting on eggs right now, correct? If I were you, and probably what any Avian Vet would tell you to do would be to not allow the eggs to hatch, because there is no way that either of these birds can feed babies at all, let alone for weeks and weeks. They aren't digesting adequate nutrition for themselves right now, so they certainly can't feed babies. I'm actually very confident and comfortable saying that if you allow the eggs to hatch and they have to feed babies, that both the parents and the babies are probably going to die of starvation/malnutrition. And I don't know if you've every hand-raised/hand-fed baby birds before, or if you have a proper Brooder to house them in, etc., but it's so difficult hand-feeding baby birds during their first 2 weeks of life, they typically die of either Aspiration of formula into their lungs, which is an instant death with babies only 1-2 weeks old due to their small lung size, or they die in a matter of days due to malnourishment or due to infections because their immune systems are not developed until they are about 2 weeks old, which is why breeders wait to pull them from the nest-box to hand-raise them until they are at least 2 weeks old, and no older than 3 weeks old...You could certainly try to pull them from the nest-box as they hatch, put them in a Brooder with an ambient temperature between 90-95 degrees F until their down-feathers grow in, then the ambient temperature can be dropped to between 80-85 degrees F until their mature feathers all grow-in, then they move into their first Weaning/Starter Cage...You'd also have to have all the hand-feeding supplies, including a cooking/candy thermometer with a metal probe so that you can ensure that the formula you're feeding them is ALWAYS between 104 degrees F and 110 degrees F, any cooler and they will die of a Fungal infection, any hotter and their Crops will burn...You'll need a commercial hand-feeding formula for baby birds, such as Kaytee or Roudybush, along with an eye-dropper to feed them for the first 2 weeks, then you can switch-over to an oral-syringe. But you have to be extremely careful to only put droplets of formula on their tongues or into their beaks and let them swallow it slowly, because it's just so easy to aspirate the formula into their lungs when they are so little...
Either way I wouldn't even attempt allowing this pair to feed the babies, because their chance of shedding any Viral disease that they have and surviving it become slim to none if they are feeding babies, and again the babies will also die because the food they will be feeding them will not be digested, so the babies won't be getting any nutrition either...If I were you, and I'm saying this as a breeder/hand-raiser of over 20 years, but I would probably try to hand-raise/hand-feed the babies from the moment they hatch, but ONLY if you have the equipment and supplies that you need, because if you don't have a Brooder or at least a homemade Brooder that can keep the chicks at the adequate ambient temperature then they don't have a chance...So if you have all of the necessary equipment to house and feed them, then I'd pull each chick as it hatches and give it a try, because allowing the parents to feed them is going to end horribly for both the parents and the babies.
As far as the parents go, all you can do is "Supportive Care", which means keeping them very well-hydrated (if you have access to an electrolyte supplement like Pedialyte that would be best, but if not then water is fine), and you need to start feeding them nutrient-dense food that is easy to digest and process...Seeds obviously aren't going to work, they are getting no nutrition at all from them when they pass whole an still in the shells...As Itzjbean already mentioned, NO MORE BREAD!!! It's nothing but sugar/carbs and it has absolutely no nutritional value to your birds...
You need to start feeding them high protein, high calorie food that is safe for them and that is easy to digest...Cooked/Scrambled Eggs are very good, you can try to grind the actual Egg-Shells into a very fine powder (mortar and pestle works best) and mix the powdered shells into the liquid eggs and then cook them, so they're getting the added Calcium, however if you don't get the shells ground into a powder that will mix well into the eggs, then the egg shell bits will just pass right out in their droppings just like the seeds do...But cooked Eggs are a good start...
***Do you have access to a commercially-sold hand-feeding formula for baby birds, such as Kaytee or Roudybush? Kaytee is probably the most-commonly sold hand-feeding formula for baby birds, and you can also buy it online...You'll not only need this if you're going to attempt to hand-feed the babies, BUT IT'S ALSO THE BEST THING YOU CAN FEED THE TWO PARENTS RIGHT NOW AS WELL...Baby bird hand-feeding formula is very nutrient rich, with lots of protein, fat, calories, and all of the amino acids/digestive enzymes added-in, along with added Probiotics, and these are going to help your birds to digest their food and absorb the nutrition for it...
If you lived in the US I would tell you to start adding a Digestive-Enzyme supplement to ALL of the food you feed the parents, because this is typically what an Avian Vet is going to suggest for birds with PDD or ABV that are not digesting their food...They are going to put them on a diet of either baby-bird hand-feeding formula multiple times a day, or a high-calorie Veterinary supplemental formula for birds, such as Emerald Critical Care. Both of these are not only very nutrient-dense and contain a ton of protein, fat, carbs, etc., but they also both contain added Digestive-Enzymes, Pre-Biotics, and Probiotics to help their bodies to digest everything they eat. You can buy supplements here made for people that are a tablet that contains both a mix of Probiotics and Digestive Enzymes that you can crush into a powder...I don't know what you have available to you or if you can order things online. If you can, then you need to order a good amount of baby-bird handfeeding formula, preferably the Kaytee Exact "High-Fat" Hand-Feeding Formula, as it contains everything that they need to sustain them while they hopefully shed the PDD or ABV virus...So you'd need to feed both of them multiple times a day with an oral syringe (if you had an Avian Vet they would "Tube-Feed" or "Crop-Feed" them with a special Crop-Needle, but you can't do that if you don't know how and don't have a Crop-Needle)...So you'll have to hand-feed the parents the formula multiple times a day just like you would the babies...Otherwise the aren't going to get enough nutrition and they are going to start losing weight quickly...