I'd say this is just a behavioural phase, especially as you have 2 dogs doing the same thing.
I don;t think you need a vet, but possibly, a trainer for some advice (look for someone with a CPDT or Stilwell Positively qualification). As others say -- 99.9999% of cavaliers need discouragement from overeating rather than encouragement to eat! lol
You might try feeding in separate rooms. Raising two puppies together takes some extra work to avoid them remaining so closely bonded that they ignore the humans and just focus on each other a lot of the time. They can develop behaviour issues where they reinforce each other (which may well be the issue with feeding now), and they can get problems with separation anxiety if they have to be separated.
So first off I'd try to give each dog separate time every day, including a walk on his own and so forth, and feed them separately.
The other thing I'd be wondering here is: are you doing anything at all to try and encourage them to eat (wrong thing to do) or just ignoring them (right thing to do)? Do you leave their food down all day til they eat, or give them a set period of just 10-15 minutes before calmly removing the food til the next scheduled feeding? Many dogs start to play this kind of food game to get more interaction with their owner and removing the food removes the point of the 'game' for them.
So: I'd feed them in two separate rooms, giving them 10-15 minutes to eat on their own with no interaction from you and no talking to them or looking in on them etc. After 10 minutes remove the bowl without any comment or fuss and store away til the next scheduled feeding and NO treats in between. Dogs really do not need to be fed store bought treats, which are high in calories, anyway, more than a couple of times a day at most, if at all. A small breed like a cavalier --also prone to weight problems -- should not get more than 1-2 small treats daily. Try giving them apple slices or slices of sweet bell pepper slices instead as these are healthier and with few calories for the dog
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Most dogs will quickly learn (within 48 hours) that now, they either eat or there's no food -- and the bwhaviour issue is solved.
I cannot stress enough how important it is for cavaliers not to get on a cycle of eating lots of treats. The single thing that will hasten the death of these dogs is being even slightly overweight, as they almost all will develop heart disease and just a few ounces extra on a small dog makes the heart work harder causing more stress (setting aside all the other issues being overweight causes for an animal -- once study showed being a few pounds overweight cut up to 4 years off the life of labradors so it is quite a serious issue).
Try splitting them and feeding them on a strict schedule and see if that works and let us know what happens! PS dogs will NOT starve themselves to death so I very much doubt there's any health issue -- these things are generally entirely short term behaviour problems but they do need to be nipped in the bud right away or can become very difficult... I know of owners who insist their dog must be hand fed or spoon fed.Mainly this means the dog has insisted the owner do this and the owner is being dictated to by the dog... !