Hello,
I'm so sorry for your loss but I urge you not to get more dogs until you have identified and removed the source of, what I agree, sounds like the effects of poison. Acute diabetes presents with similar symptoms but seems unlikely in two dogs at the same time in the same place unless a totally innapropriate diet was given, and even then it would of had to have been exteme.
I suggest you tactfully ask neighbours if they have had pest problems that warranted putting poisoned bait down or try phoning the local pest control agencies and ask them if they have been active in your area.
The other thing that springs to mind is possible lead poisoning. This builds up in the system with few noticeable effects, especially in puppies where any "odd" behaviour can be put down to playfulness, until a fatal level is reached. Then death is often swift and distressing and unexpected.
The source of such poisoning is sometimes very hard to find but try having your water pipes tested, important as it will affect you too! Any painted surface, outside or in must be suspect too.
Some comfort may be that usually lead affected dogs become sensitive to light and many become hydrophobic, but many become addicted to the source of the lead and take on such large amounts at a time that they become critical with little or no warning.
I hope that if I haven't given you an answer for what has happened then at least I have ruled something out. Best wishes, my thoughts are with you.