My heart goes out to you; I can hardly imagine anything more devastating than losing TWO young foals in this manner, with NO indication of anything wrong! You are certainly doing the right thing by having the necropsy-in hopes it will provide some answers.
Years ago, I lost a youngster at a few weeks due to PDA-Peyton Ductis Arteriosis, where an opening in the heart area(don't recall all the details, just know this can happen in humans and dogs, also....)doesn't close soon after birth as it should, and results in death if not repaired/repairable-I know someone whose son had to have surgery for it; more recently, I have been told that it is 'now' treatable by medication--in humans, not sure about other mammals. In hindsight, we realized that the colt had seemed too 'tired' after exercise-but it was late June and hot, and he'd run around when turned out, so we just thought he'd gotten too hot--he seemed normal in every other way. I was gone to Estes Park when he was lost, but my late husband HAD noticed him in distress and got the vet here, but she couldn't help the colt by then. If memory serves, a vet should be able to tell by listening to the heart w/ a stethoscope if there is such a problem. Can't imagine it would occur with two, though.
My only suggestion would be as others have said-perhaps keep the newest colt out of the area where the others were, at least until you can investigate further?? Wish I could be of more help...my thoughts are with you in your grief.
Margo