Just ruined the ending
No couple dies at same due to natural causes no matter how much they love each other. To be fair, due to his heart attack Noah could've died that night but he seemed he was recovering, but no one dies of dementia so Ellie dying at the exact same night it's *beep* Yes it IS possible but the chances are very low.
Here's what most likely happened: they've made a suicide pact and swallowed a bunch of pill they got. I just ruined a movie I actually enjoyed for myself.