Not recommended, but in an emergency they could re-use the needles. Just make sure to have 'his' and 'hers'.
Presumably a couple that's been married more than a couple years has shared pretty much all their bodily fluids other than the ones that take serious digging to get to and they share air and probably saliva daily, so cross-infection isn't a serious worry unless one is known to have something contagious by blood contact at the time. Cross contamination of the meds could be minimized (not eliminated, but how much is acceptable when the alternative is one person not getting their meds at all?) with a rinse, but really, if you need daily injections, I can't see going farther than the corner store with less than three needles in the car. I tend to store current-use stock of critical supplies in the car specifically because then I have it if I decide on the spur of the moment to stay with a friend, or I get stuck in another town due to weather or breakdown.
More troubling would be the lack of food and/or water. As you mention, a guy in his 80s probably doesn't need much of a push to die.
Yeah; I also generally keep a case of bottled water in the trunk and at least a decent meal worth of durable food even just for driving around town. Gandhi fasted 21 days when he was in his 70s, but I don't recall him doing a lot of hikes during that time. Bugging out from almost anywhere I normally drive, I can hike to civilization in any navigable conditions in a few hours. Traveling during breaks in a storm or trudging through snow/ice from the most remote spots within a 4 hour drive, I could see taking up to a full 8-12 hour day to get to somewhere I can beg shelter and food. (And that assumes both following roads and taking the worst possible direction from my breakdown point - heading out away from a town just out of sight around the next bend, for example - though not getting utterly stupid; there will be direction signs and traffic once I get to a major road.) Obviously, a trip out of my area of familiarity would call for some extra preparation; when I drove to the Rio Grande Valley, (six degrees of latitude difference) I looked over the route, noted a few long gaps between towns, doubled up on the food and carried a 5 gallon gas can in addition to my usual 2 gallon. Climate was a bit different, but well within the range of what I normally deal with here, (Basically, the heat and humidity there in April wouldn't be uncommon here in August if wet summers were more common here. It was abnormally wet for them, though.) and the flora and fauna weren't different enough to cause any issues with recognizing some basic edibles.
Sitting tight and sheltering in the car, 3 days without food isn't much of a concern. Building a base camp and/or tending injuries, I'd have to eat something within two days to avoid hypoglycemia-induced confusion and other symptoms, but that one decent meal should be able to stretch to 3-4 days worth of staving off the problematic symptoms. Even in the dead of winter, there are food sources around here that would maintain me at "reasonably safe" calorie count for a while, (I can stand to burn some fat reserves, but generally you need to take in 1,200 calories a day to keep the metabolic processes on the "happy" side rather than the "oh crap, we're starving and need to burn protein instead" track. While you can live a couple of weeks like that, in an indefinite survival situation, it's a bad idea to get there because you're going to need that protein in your muscles to find food, maintain your shelter, etc.)
