I always afraid to put them together because i am afraid if one get sick then can contaminate another one. I always have Aquarium salt and Epsom salt and medications at home just in case , but luckily never use it yet. I don't have any bettas less than 2.3-2.5 years old. My oldest one about approx will be soon i think 4 years old
Also if you had 2 bettas died i think you need to change the water in case they had bacterial or fungal infection. Or just keep eye on them make sure they will not develop any physical symptoms.
I also learned that the warmer the water the more chance of the infections. So i think if your water very warm then you need more water changes. :)