I'm so sorry to hear about Jarvis and Harvey. It must be hard to lose them one after the other like that, but I don't think it was anything you did or didn't do. The fact that they were both getting on in years probably made it much more likely that you would lose them in close proximity to each other. People often have pets that are the same age or close to the same age and it oftens happens that they lose them within months of each other (I work at a vets so see it quite a lot, so much so that I have made a point of having at least 4 years age difference between each of my pets to hope to avoid it). I think it's just one of those things that can happen, that doesnt help I know but I really don't think it was stress etc, and probably was just a heart attack and there was unfortunately nothing you can do.
I know it's hard not to question why what happened happened but I'd recommend this site:
http://www.pet-loss-matters.com
Hope this helps a bit at least.