The kittens might still have heart murmurs. Maybe the other vets hearing was better, I am sure he wouldn't lie and make it up. I've never heard of a murmur going away on its own-and in BOTH kitties on top of that. What this tells me, is that the heart murmur must be very very slight. And, with a slight murmur in both of the babies, it is very likely to be a genetic thing. If only one of the babies had a heart murmur, I would think there was an abnormality that should be looked into.
I, personally, if they were mine, I wouldn't even worry about it especially if they were playing and eating normally. There are people that are born with heart murmurs, too. My ex-boyfriend had one and didn't even know it-I found it when I layed my head on his chest. I asked him, how long he's had the heart murmur. He was totally clueless. I made him call his mom, and she confirmed it. He was fine and the heart "problem" didn't interfere with his alcoholism. LOL.
You really have nothing to worry about! I think they'll be fine.
I would make sure the kittens don't have heart murmur. I'd go with a 3rd vet, even better, I'd go with a specialist. Right now, you've been told yes and no, so there is a 50% chance that they don't and 50% chance that they do have it.
Some cats have had heart murmurs all their lives without any apparent problems. The only really good way to identify the cause of a heart murmur in cats is an ultrasound exam. For this reason, it is best to have this examination done by a veterinary cardiology specialist. I'd go with the specialist. You didn't mention an ultrasound, so you may want to start thinking about it.
X-rays are not helpful in determining the existence of a heart murmur. I'm telling you this because some vets like to do unnecessary tests to keep the business going. An ultrasound is the only really good way to determine the severity of a heart
murmur in a cat. Ask for a referral at the vet clinic. I'm sure they'll gladly help.
Good Luck.